Empty Metadata fields in new model throw exception

When editing the Metadata information of a new trained model, if the Description and Authors fields are left empty, it will throw the following error into the console (see below).

It only happens in a new trained model. If the same thing is done in a model that already had metadata in it, no error is shown.

```
[ERROR] Module threw exception
java.lang.NullPointerException
at net.imagej.modelzoo.consumer.command.ModelArchiveEditMetaDataCommand.stringToList(ModelArchiveEditMetaDataCommand.java:102)
at net.imagej.modelzoo.consumer.command.ModelArchiveEditMetaDataCommand.run(ModelArchiveEditMetaDataCommand.java:70)
at org.scijava.command.CommandModule.run(CommandModule.java:196)
at org.scijava.module.ModuleRunner.run(ModuleRunner.java:165)
at org.scijava.module.ModuleRunner.call(ModuleRunner.java:124)
at org.scijava.module.ModuleRunner.call(ModuleRunner.java:63)
at org.scijava.thread.DefaultThreadService.lambda$wrap$2(DefaultThreadService.java:225)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(FutureTask.java:122)
at java.util.concurrent.FutureTask.get(FutureTask.java:192)
at net.imagej.modelzoo.display.SwingModelArchiveDisplayViewer.editMetaData(SwingModelArchiveDisplayViewer.java:458)
at net.imagej.modelzoo.display.SwingModelArchiveDisplayViewer.access$100(SwingModelArchiveDisplayViewer.java:104)
at net.imagej.modelzoo.display.SwingModelArchiveDisplayViewer$3.lambda$actionPerformed$0(SwingModelArchiveDisplayViewer.java:227)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
at net.imagej.modelzoo.consumer.command.ModelArchiveEditMetaDataCommand.stringToList(ModelArchiveEditMetaDataCommand.java:102)
at net.imagej.modelzoo.consumer.command.ModelArchiveEditMetaDataCommand.run(ModelArchiveEditMetaDataCommand.java:70)
at org.scijava.command.CommandModule.run(CommandModule.java:196)
at org.scijava.module.ModuleRunner.run(ModuleRunner.java:165)
at org.scijava.module.ModuleRunner.call(ModuleRunner.java:124)
at org.scijava.module.ModuleRunner.call(ModuleRunner.java:63)
at org.scijava.thread.DefaultThreadService.lambda$wrap$2(DefaultThreadService.java:225)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
... 1 more
```

Start in ModelArchiveEditMetaDataCommand.java, especially run at line 70 and stringToList at line 102, then reproduce the issue by editing a new trained model with empty Description and Authors fields. Done means saving that metadata no longer throws the reported NullPointerException; verify the existing-model case still works.

Written by the indexing model from the issue text.