RFR: 7231: JMC createReport for JMC8 Automated Analysis fails to evaluate several rules

[Suchita Chaturvedi]

This PR addresses the NullPointerException issue in core module for the following rules:

1. GCs Caused by System.gc()
2. GCs Caused by Heap Inspection
3. GC Stall
4. String Deduplication
5. GCs Caused by GC Locker

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Commit messages:
 - 7231: JMC createReport for JMC8 Automated Analysis fails to evaluate several rules

Changes: https://git.openjdk.java.net/jmc/pull/302/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=302&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7231
  Stats: 99 lines in 5 files changed: 33 ins; 12 del; 54 mod
  Patch: https://git.openjdk.java.net/jmc/pull/302.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/302/head:pull/302

PR: https://git.openjdk.java.net/jmc/pull/302