Handle potentially null moduleName in restoreModule

[ThanHenderson]

When VM snapshotting is enabled, restore runs need to restore snapshotted modules.

The restoration procedure in java11vmi.c::restoreModule loops over all classes loaded from the persisted systemClassLoader, then performs a string compare to identify whether a classes module is the java.base module.

There is an issue that I've seen in JDK21 (but likely affects other versions too), that a clazz->module->moduleName is null, but this case isn't handled (results in a seg fault here).

Unnamed module's module names are set to NULL [1, 2], so if the walker over classes from systemClassLoader returns a class whose module is an unnamed module, the problematic code path will result in a failure.