RFR: 8259622: TreeMap.computeIfAbsent deviates from spec

[Tagir F.Valeev]

Handle TreeMap::computeIfAbsent when previous mapping with null value existed (in this case spec requires to overwrite the existing mapping)

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Commit messages:
 - 8259622: TreeMap.computeIfAbsent deviates from spec

Changes: https://git.openjdk.java.net/jdk/pull/2058/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2058&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259622
  Stats: 24 lines in 2 files changed: 22 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2058.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2058/head:pull/2058

PR: https://git.openjdk.java.net/jdk/pull/2058