Running `jdeps` with `--check <module>` and "missing" modules, i.e. some required modules are not available on the module path, yields a NullPointerException.

See https://github.com/joel-costigliola/assertj-core/issues/1504#issuecomment-501956593 and previous comments for details.

jdeps --multi-release 9 --module-path target/assertj-core-3.13.0-SNAPSHOT.jar --check org.assertj.core

org.assertj.core (file:///[...]/assertj-core/target/assertj-core-3.13.0-SNAPSHOT.jar)

  [Module descriptor]
    requires mandated java.base;
    requires java.instrument;

  [Suggested module descriptor for org.assertj.core]
    requires mandated java.base;
    requires java.instrument;
    requires java.logging;
    requires java.management;
    requires java.xml;
    requires transitive unnamed;

Exception in thread "main" java.lang.NullPointerException
        at jdk.jdeps/com.sun.tools.jdeps.ModuleGraphBuilder.requiresTransitive(ModuleGraphBuilder.java:124)
        at jdk.jdeps/com.sun.tools.jdeps.ModuleGraphBuilder.buildGraph(ModuleGraphBuilder.java:110)
        at jdk.jdeps/com.sun.tools.jdeps.ModuleGraphBuilder.buildGraph(ModuleGraphBuilder.java:80)
        at jdk.jdeps/com.sun.tools.jdeps.ModuleAnalyzer$ModuleDeps.buildReducedGraph(ModuleAnalyzer.java:182)
        at jdk.jdeps/com.sun.tools.jdeps.ModuleAnalyzer$ModuleDeps.reduced(ModuleAnalyzer.java:195)
        at jdk.jdeps/com.sun.tools.jdeps.ModuleAnalyzer$ModuleDeps.analyzeDeps(ModuleAnalyzer.java:215)
        at jdk.jdeps/com.sun.tools.jdeps.ModuleAnalyzer.lambda$run$3(ModuleAnalyzer.java:92)
        at java.base/java.util.HashMap$Values.forEach(HashMap.java:976)
        at jdk.jdeps/com.sun.tools.jdeps.ModuleAnalyzer.run(ModuleAnalyzer.java:88)
        at jdk.jdeps/com.sun.tools.jdeps.JdepsTask$CheckModuleDeps.run(JdepsTask.java:968)
        at jdk.jdeps/com.sun.tools.jdeps.JdepsTask.run(JdepsTask.java:560)
        at jdk.jdeps/com.sun.tools.jdeps.JdepsTask.run(JdepsTask.java:519)
        at jdk.jdeps/com.sun.tools.jdeps.Main.main(Main.java:49)