RFR: 8280743: HSDB "Monitor Cache Dump" command might throw NPE

[Chris Plummer]

ObjectMonitor.object() can be null so we need to defend against it. This bug was discovered by code inspection while working on [JDK-8280555](https://bugs.openjdk.org/browse/JDK-8280555).  We have no test for this, and I'm not sure how to reproduce this with HSDB like the [JDK-8280555](https://bugs.openjdk.org/browse/JDK-8280555) test did.  I did at least verify that the HSDB "Monitor Cache Dump" feature still works after this fix, although none of the monitors had a null object.

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Commit messages:
 - Check for null mon.object()

Changes: https://git.openjdk.org/jdk/pull/15369/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15369&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8280743
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15369.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15369/head:pull/15369

PR: https://git.openjdk.org/jdk/pull/15369