RFR: 8332039: Cannot invoke "com.sun.source.util.DocTreePath.getTreePath()" because "path" is null

[Hannes Wallnöfer]

Please review a patch to fix a NPE thrown when a `@since` tag inherited by a nested class contains a nested inline tag. The solution is to make `CommentHelper.getDocTreePath(DocTree)` able to handle block tags inherited by nested classes.

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Commit messages:
 - JDK-8332039: Cannot invoke "com.sun.source.util.DocTreePath.getTreePath()" because "path" is null

Changes: https://git.openjdk.org/jdk/pull/19363/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19363&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332039
  Stats: 68 lines in 2 files changed: 49 ins; 3 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/19363.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19363/head:pull/19363

PR: https://git.openjdk.org/jdk/pull/19363