RFR: 8300840: Update test runtime/Thread/StopAtExit.java to include Null Pointer Exception as one of the allowed exceptions

[Afshin Zafari]

In the test, NPE is caught with no action.
### Test
mach5 tier1-5

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Commit messages:
 - 8300840: Update test runtime/Thread/StopAtExit.java to include Null Pointer Exception as one of the allowed exceptions

Changes: https://git.openjdk.org/jdk/pull/13013/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13013&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300840
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13013.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13013/head:pull/13013

PR: https://git.openjdk.org/jdk/pull/13013