RFR: 8287602: (fs) Avoid redundant HashMap.containsKey call in MimeTypesFileTypeDetector.putIfAbsent

[Andrey Turbanov]

Only non-null values are put into `Map<String,String> mimeTypeMap`. It means, we can replace `containsKey`+`put` calls with single `putIfAbsent` call. It makes code a bit cleaner and faster.

https://github.com/openjdk/jdk/blob/3d2d039538b906cedd9188ed94b7ba55c275ff7f/src/java.base/unix/classes/sun/nio/fs/MimeTypesFileTypeDetector.java#L189-L196

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

Changes: https://git.openjdk.java.net/jdk/pull/8941/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8941&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287602
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8941.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8941/head:pull/8941

PR: https://git.openjdk.java.net/jdk/pull/8941