RFR: 8305538: Avoid redundant HashMap.containsKey call in ModuleDescriptor.Builder

[Andrey Turbanov]

`Map.containsKey` call is sometimes unnecessary, when it's known that `Map` doesn't contain `null` values.
Instead of pair containsKey+put we can use putIfAbsent and compare result with null.
Result code is shorter and a bit faster.

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Commit messages:
 - [PATCH] Avoid redundant HashMap.containsKey call in ModuleDescriptor

Changes: https://git.openjdk.org/jdk/pull/13288/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13288&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305538
  Stats: 9 lines in 1 file changed: 0 ins; 2 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/13288.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13288/head:pull/13288

PR: https://git.openjdk.org/jdk/pull/13288