RFR: 8277120: Use Optional.isEmpty instead of !Optional.isPresent in java.net.http

[Andrey Turbanov]

I propose to replace usages of !Optional.isPresent() with Optional.isEmpty method.
It's makes code a bit easier to read.

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Commit messages:
 - [PATCH] Use Optional.isEmpty instead of !Optional.isPresent in java.net.http

Changes: https://git.openjdk.java.net/jdk/pull/5985/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5985&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277120
  Stats: 13 lines in 5 files changed: 0 ins; 3 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5985.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5985/head:pull/5985

PR: https://git.openjdk.java.net/jdk/pull/5985