RFR: 8308167: SequencedMap::firstEntry throws NPE when first entry has null key or value

[Stuart Marks]

Create and use new NullableKeyValueHolder class to accommodate map entries whose key or value might be null.

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Commit messages:
 - 8308167: SequencedMap::firstEntry throws NPE when first entry has null key or value

Changes: https://git.openjdk.org/jdk/pull/14278/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14278&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308167
  Stats: 214 lines in 4 files changed: 208 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/14278.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14278/head:pull/14278

PR: https://git.openjdk.org/jdk/pull/14278