[jdk21u-dev] RFR: 8317360: Missing null checks in JfrCheckpointManager and JfrStringPool initialization routines

[Neethu Prasad]

**Notes**

Backport of [JDK-8317360](https://bugs.openjdk.org/browse/JDK-8317360)

**Verification**

* jdk_jfr, Tier1 & Tier 2 tests.

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Commit messages:
 - Backport cf4ede0e544fbd758cc9e93c695fb19fb7da0f9a

Changes: https://git.openjdk.org/jdk21u-dev/pull/737/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=737&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317360
  Stats: 6 lines in 2 files changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/737.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/737/head:pull/737

PR: https://git.openjdk.org/jdk21u-dev/pull/737