Fix null context class loader handling

[Aayush10016]

Summary

• avoid dereferencing a null thread context class loader
• return the existing unresolved-location result and diagnostic instead of throwing
• add a regression test that restores the original context class loader after verification

Root cause

ClasspathResourceLocatorStrategy.resolve called getResource unconditionally on Thread.currentThread().getContextClassLoader(). The context class loader is allowed to be null in some runtime environments, which caused a NullPointerException.

Testing

• mvn -Dtest=ClasspathResourceLocatorStrategyTest test
  • 5 tests passed
• mvn -Prun-its verify
  • 80 tests passed
  • Checkstyle, Spotless, Apache RAT, packaging, and the integration-test profile succeeded

Fixes #90

[elharo]

This isn't right. If getContextClassLoader() returns null, you should use the system classloader.

[elharo]

mh --> messageHolder