RFR: 8364434: Inconsistent BufferedContext state after GC

[Nikita Gubarkov]

For "true" null objects, reset the ref itself to null. Non-null ref with null content means that the object was GC'ed. GC'ed state always behaves as not-equal to the new one, causing corresponding ops to be written into RQ.

Although I could not find practical scenarios where refs other than `validPaintRef` could cause problems, this is generally fragile and potentially problematic for any state object kept in weak ref. Therefore I changed the usage of all weak refs in the same way.

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Commit messages:
 - 8364434: Inconsistent BufferedContext state after GC

Changes: https://git.openjdk.org/jdk/pull/26576/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26576&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364434
  Stats: 37 lines in 1 file changed: 12 ins; 2 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/26576.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26576/head:pull/26576

PR: https://git.openjdk.org/jdk/pull/26576