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