NPE during display-discovered-jdk-toolchains goal in ToolchainDiscoverer.discoverToolchains()

### Affected version

3.3.0

### Bug description

This is similar to #113 and #119 , the cause just moves around.

This time it's here: https://github.com/apache/maven-toolchains-plugin/blob/44956705ba6ba0ff6a750cdae1878f5bcf15e167/src/main/java/org/apache/maven/plugins/toolchain/jdk/ToolchainDiscoverer.java#L152-L155

`ToolchainModel tc` is set to `null` on line 152, and dereferenced on line 155.

I have an install of JDK 1.5 at /usr/lib/jvm/jdk1.5.0_22/ which I use to trigger these issues, should you want to replicate.

Command issued `mvn -X org.apache.maven.plugins:maven-toolchains-plugin:3.3.0:display-discovered-jdk-toolchains`

Output:
```
[DEBUG] Loading mojo org.apache.maven.plugins:maven-toolchains-plugin:3.3.0:display-discovered-jdk-toolchains from plugin realm ClassRealm[plugin>org.apache.maven.plugins:maven-toolchains-plugin:3.3.0, parent: sun.misc.Launcher$AppClassLoader@7852e922]
[DEBUG] Configuring mojo execution 'org.apache.maven.plugins:maven-toolchains-plugin:3.3.0:display-discovered-jdk-toolchains:default-cli' with basic configurator -->
[DEBUG] (f) comparator = lts,current,env,version,vendor
[DEBUG] -- end configuration --
[INFO] Found 3 possible jdks: [/usr/lib/jvm/adoptopenjdk-11-hotspot-amd64, /usr/lib/jvm/jdk1.5.0_22, /usr/lib/jvm/adoptopenjdk-8-hotspot-amd64]
[DEBUG] JDK toolchain discovered at /usr/lib/jvm/jdk1.5.0_22 will be ignored: could not obtain java.version
[WARNING] Error discovering toolchains: java.lang.NullPointerException
java.lang.NullPointerException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0 (Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance (NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance (DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance (Constructor.java:423)
at java.util.concurrent.ForkJoinTask.getThrowableException (ForkJoinTask.java:598)
at java.util.concurrent.ForkJoinTask.reportException (ForkJoinTask.java:677)
at java.util.concurrent.ForkJoinTask.invoke (ForkJoinTask.java:735)
at java.util.stream.Nodes.collect (Nodes.java:328)
at java.util.stream.ReferencePipeline.evaluateToNode (ReferencePipeline.java:109)
at java.util.stream.AbstractPipeline.evaluate (AbstractPipeline.java:541)
at java.util.stream.SortedOps$OfRef.opEvaluateParallel (SortedOps.java:157)
at java.util.stream.AbstractPipeline.opEvaluateParallelLazy (AbstractPipeline.java:705)
at java.util.stream.AbstractPipeline.sourceSpliterator (AbstractPipeline.java:432)
at java.util.stream.AbstractPipeline.evaluate (AbstractPipeline.java:233)
at java.util.stream.ReferencePipeline.collect (ReferencePipeline.java:566)
at org.apache.maven.plugins.toolchain.jdk.ToolchainDiscoverer.discoverToolchains (ToolchainDiscoverer.java:162)
at org.apache.maven.plugins.toolchain.jdk.DisplayDiscoveredJdkToolchainsMojo.execute (DisplayDiscoveredJdkToolchainsMojo.java:65)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:126)
at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2 (MojoExecutor.java:328)
at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute (MojoExecutor.java:316)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:212)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:174)
at org.apache.maven.lifecycle.internal.MojoExecutor.access$000 (MojoExecutor.java:75)
at org.apache.maven.lifecycle.internal.MojoExecutor$1.run (MojoExecutor.java:162)
at org.apache.maven.plugin.DefaultMojosExecutionStrategy.execute (DefaultMojosExecutionStrategy.java:39)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:159)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:105)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:73)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:53)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:118)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:261)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:173)
at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:101)
at org.apache.maven.cli.MavenCli.execute (MavenCli.java:906)
at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:283)
at org.apache.maven.cli.MavenCli.main (MavenCli.java:206)
at sun.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke (Method.java:498)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:283)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:226)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:407)
at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:348)
Caused by: java.lang.NullPointerException
at org.apache.maven.plugins.toolchain.jdk.ToolchainDiscoverer.lambda$discoverToolchains$4 (ToolchainDiscoverer.java:155)
at java.util.stream.ReferencePipeline$3$1.accept (ReferencePipeline.java:193)
at java.util.HashMap$KeySpliterator.forEachRemaining (HashMap.java:1556)
at java.util.stream.AbstractPipeline.copyInto (AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:472)
at java.util.stream.Nodes$CollectorTask.doLeaf (Nodes.java:2183)
at java.util.stream.Nodes$CollectorTask.doLeaf (Nodes.java:2149)
at java.util.stream.AbstractTask.compute (AbstractTask.java:327)
at java.util.concurrent.CountedCompleter.exec (CountedCompleter.java:731)
at java.util.concurrent.ForkJoinTask.doExec (ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask (ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker (ForkJoinPool.java:1692)
at java.util.concurrent.ForkJoinWorkerThread.run (ForkJoinWorkerThread.java:175)
[INFO] Discovered 0 JDK toolchains:
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 0.487 s
[INFO] Finished at: 2026-07-28T20:25:55-05:00
[INFO] ------------------------------------------------------------------------
```

What I think should happen: even if the version can't be determined for a potential JDK, the others should be found.

I also see a lot of Java streams, parallelStream, filter and mapping in this code, but also returning null and (missed) null checks. Why not embrace Optional if you're going to use the other functional patterns? No more nulls, no more NPEs, return Optional.empty().

Start in src/main/java/org/apache/maven/plugins/toolchain/jdk/ToolchainDiscoverer.java at discoverToolchains(), especially lines 152-155, and reproduce the issue with the display-discovered-jdk-toolchains Maven command and an unusable JDK installation. Done means a JDK whose version cannot be obtained is ignored without an NPE, while the other discovered JDK toolchains are still reported.

Written by the indexing model from the issue text.