NPE Parsing a workflow

Dockstore is failing to register this [workflow](https://github.com/nebiolabs/neb-galaxy-workflows/blob/f3ab079e9b365794727c13a5da3daea064a26513/nebnext-immune-sequencing/presto_immune_sequencing_v3.20.ga) from this repo: https://github.com/nebiolabs/neb-galaxy-workflows

I tried debugging it a little, and this plugin is throwing an NPE, so there's not even an error message that the end-user sees.

Here's the stack trace I see when running in the plugin locally (I added a test in my local repo, hence the `testPrestoImmuneValidation` method which doesn't exist in this repo:

```
java.lang.NullPointerException
at org.galaxyproject.gxformat2.v19_09.utils.LoadingOptions.expandUrl(LoadingOptions.java:93)
at org.galaxyproject.gxformat2.v19_09.utils.UriLoader.expandUrl(UriLoader.java:26)
at org.galaxyproject.gxformat2.v19_09.utils.UriLoader.load(UriLoader.java:48)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.load(Loader.java:16)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.loadField(Loader.java:85)
at org.galaxyproject.gxformat2.v19_09.WorkflowStepInputImpl.(WorkflowStepInputImpl.java:149)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
at org.galaxyproject.gxformat2.v19_09.utils.RecordLoader.load(RecordLoader.java:23)
at org.galaxyproject.gxformat2.v19_09.utils.RecordLoader.load(RecordLoader.java:6)
at org.galaxyproject.gxformat2.v19_09.utils.UnionLoader.load(UnionLoader.java:26)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.load(Loader.java:16)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.loadField(Loader.java:85)
at org.galaxyproject.gxformat2.v19_09.utils.ArrayLoader.load(ArrayLoader.java:27)
at org.galaxyproject.gxformat2.v19_09.utils.ArrayLoader.load(ArrayLoader.java:6)
at org.galaxyproject.gxformat2.v19_09.utils.OptionalLoader.load(OptionalLoader.java:20)
at org.galaxyproject.gxformat2.v19_09.utils.OptionalLoader.load(OptionalLoader.java:5)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.load(Loader.java:16)
at org.galaxyproject.gxformat2.v19_09.utils.IdMapLoader.load(IdMapLoader.java:51)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.load(Loader.java:16)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.loadField(Loader.java:85)
at org.galaxyproject.gxformat2.v19_09.WorkflowStepImpl.(WorkflowStepImpl.java:476)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
at org.galaxyproject.gxformat2.v19_09.utils.RecordLoader.load(RecordLoader.java:23)
at org.galaxyproject.gxformat2.v19_09.utils.RecordLoader.load(RecordLoader.java:6)
at org.galaxyproject.gxformat2.v19_09.utils.UnionLoader.load(UnionLoader.java:26)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.load(Loader.java:16)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.loadField(Loader.java:85)
at org.galaxyproject.gxformat2.v19_09.utils.ArrayLoader.load(ArrayLoader.java:27)
at org.galaxyproject.gxformat2.v19_09.utils.ArrayLoader.load(ArrayLoader.java:6)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.load(Loader.java:16)
at org.galaxyproject.gxformat2.v19_09.utils.IdMapLoader.load(IdMapLoader.java:51)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.load(Loader.java:16)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.loadField(Loader.java:85)
at org.galaxyproject.gxformat2.v19_09.GalaxyWorkflowImpl.(GalaxyWorkflowImpl.java:311)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
at org.galaxyproject.gxformat2.v19_09.utils.RecordLoader.load(RecordLoader.java:23)
at org.galaxyproject.gxformat2.v19_09.utils.RecordLoader.load(RecordLoader.java:6)
at org.galaxyproject.gxformat2.v19_09.utils.UnionLoader.load(UnionLoader.java:26)
at org.galaxyproject.gxformat2.v19_09.utils.Loader.documentLoad(Loader.java:41)
at org.galaxyproject.gxformat2.v19_09.utils.RootLoader.loadDocument(RootLoader.java:18)
at org.galaxyproject.gxformat2.v19_09.utils.RootLoader.loadDocument(RootLoader.java:23)
at org.galaxyproject.gxformat2.v19_09.utils.RootLoader.loadDocument(RootLoader.java:27)
at org.galaxyproject.gxformat2.Format2Linter.lint(Format2Linter.java:10)
at org.galaxyproject.gxformat2.Lint.lint(Lint.java:20)
at org.galaxyproject.dockstore_galaxy_interface.language.GalaxyWorkflowPlugin$GalaxyWorkflowPluginImpl.validateWorkflowSet(GalaxyWorkflowPlugin.java:115)
at org.galaxyproject.dockstore_galaxy_interface.language.GalaxyWorkflowLanguagePluginTest.testPrestoImmuneValidation(GalaxyWorkflowLanguagePluginTest.java:173)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)
at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)
```

Start with LoadingOptions.expandUrl and the surrounding UriLoader.load path shown in the stack trace, then inspect GalaxyWorkflowPluginImpl.validateWorkflowSet and GalaxyWorkflowLanguagePluginTest. Reproduce the linked workflow failure and confirm that parsing no longer produces an unhandled NullPointerException and instead exposes an error to the end user.

Written by the indexing model from the issue text.