RFR: 8304541: Modules THROW_MSG_ should return nullptr instead of JNI_FALSE

[Julian Waters]

Cleanup in NPE code to return a proper nullptr instead of JNI_FALSE. JNI_FALSE expands to 0, which technically works as a null pointer here, but this becomes pretty confusing and strange when reading through the code

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Commit messages:
 - Modules THROW_MSG_ should return nullptr instead of JNI_FALSE

Changes: https://git.openjdk.org/jdk/pull/13105/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13105&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304541
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13105.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13105/head:pull/13105

PR: https://git.openjdk.org/jdk/pull/13105