RFR: 8305785: Avoid redundant HashMap.containsKey call in java.util.regex

[Andrey Turbanov]

`Pattern.namedGroups` and `Matcher.namedGroups` contains only non-null values. It means instead of separate `containsKey`+`get` calls, we can use single `HashMap.get` call and then compare result with null.
Result code is a bit simpler and faster.

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Commit messages:
 - 8305785: Avoid redundant HashMap.containsKey call in java.util.regex
 - [PATCH] Avoid redundant HashMap.containsKey call in java.util.regex{Matcher, Pattern}

Changes: https://git.openjdk.org/jdk/pull/13303/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13303&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305785
  Stats: 10 lines in 2 files changed: 3 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/13303.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13303/head:pull/13303

PR: https://git.openjdk.org/jdk/pull/13303