Cannot invoke "java.util.Map.keySet()" because "cache" is null

When using -system with a java 11 JVM while running on a java 17 jvm compilation fails with:

```
java.lang.NullPointerException: Cannot invoke "java.util.Map.keySet()" because "cache" is null
at org.eclipse.jdt.internal.compiler.batch.ClasspathJep247Jdk12.loadModules(ClasspathJep247Jdk12.java:211)
at org.eclipse.jdt.internal.compiler.batch.ClasspathJrt.initialize(ClasspathJrt.java:200)
at org.eclipse.jdt.internal.compiler.batch.ClasspathJep247.initialize(ClasspathJep247.java:118)
at org.eclipse.jdt.internal.compiler.batch.ClasspathJep247Jdk12.initialize(ClasspathJep247Jdk12.java:143)
at org.eclipse.jdt.internal.compiler.batch.FileSystem.(FileSystem.java:235)
at org.eclipse.jdt.internal.compiler.batch.Main.getLibraryAccess(Main.java:3495)
at org.eclipse.jdt.internal.compiler.batch.Main.performCompilation(Main.java:4741)
at org.eclipse.jdt.internal.compiler.batch.Main.compile(Main.java:1802)
at org.eclipse.tycho.compiler.jdt.JDTCompiler.compileInProcess(JDTCompiler.java:372)
at org.eclipse.tycho.compiler.jdt.JDTCompiler.performCompile(JDTCompiler.java:123)
at copied.org.apache.maven.plugin.AbstractCompilerMojo.execute(AbstractCompilerMojo.java:414)
at org.eclipse.tycho.compiler.AbstractOsgiCompilerMojo.doCompile(AbstractOsgiCompilerMojo.java:370)
at org.eclipse.tycho.compiler.AbstractOsgiCompilerMojo.execute(AbstractOsgiCompilerMojo.java:351)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2(MojoExecutor.java:370)
at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute(MojoExecutor.java:351)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:215)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:171)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:163)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:294)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
at org.apache.maven.cli.MavenCli.execute(MavenCli.java:960)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:293)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:196)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)
java.lang.NullPointerException: Cannot invoke "java.util.Map.keySet()" because "cache" is null
at org.eclipse.jdt.internal.compiler.batch.ClasspathJep247Jdk12.loadModules(ClasspathJep247Jdk12.java:211)
at org.eclipse.jdt.internal.compiler.batch.ClasspathJrt.initialize(ClasspathJrt.java:200)
at org.eclipse.jdt.internal.compiler.batch.ClasspathJep247.initialize(ClasspathJep247.java:118)
at org.eclipse.jdt.internal.compiler.batch.ClasspathJep247Jdk12.initialize(ClasspathJep247Jdk12.java:143)
at org.eclipse.jdt.internal.compiler.batch.FileSystem.(FileSystem.java:235)
at org.eclipse.jdt.internal.compiler.batch.Main.getLibraryAccess(Main.java:3495)
at org.eclipse.jdt.internal.compiler.batch.Main.performCompilation(Main.java:4741)
at org.eclipse.jdt.internal.compiler.batch.Main.compile(Main.java:1802)
at org.eclipse.tycho.compiler.jdt.JDTCompiler.compileInProcess(JDTCompiler.java:372)
at org.eclipse.tycho.compiler.jdt.JDTCompiler.performCompile(JDTCompiler.java:123)
at copied.org.apache.maven.plugin.AbstractCompilerMojo.execute(AbstractCompilerMojo.java:414)
at org.eclipse.tycho.compiler.AbstractOsgiCompilerMojo.doCompile(AbstractOsgiCompilerMojo.java:370)
at org.eclipse.tycho.compiler.AbstractOsgiCompilerMojo.execute(AbstractOsgiCompilerMojo.java:351)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2(MojoExecutor.java:370)
at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute(MojoExecutor.java:351)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:215)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:171)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:163)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:294)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
at org.apache.maven.cli.MavenCli.execute(MavenCli.java:960)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:293)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:196)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)
```

see also https://github.com/eclipse-tycho/tycho/issues/1354