RFR: 8252999: replace all String.equals("") usages with String.isEmpty()

[Dmitriy Dumanskiy]

**isEmpty** is faster + has less byte code + easier to read. Benchmarks could be found
  [here](https://medium.com/javarevisited/micro-optimizations-in-java-string-equals-22be19fd8416).

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Commit messages:
 - Merge branch 'master' of https://github.com/doom369/jdk into reaplce_equals_with_is_empty
 - revert change in classes that maintain jdk 1.4 compatibility
 - Improvement: replace all occurrences of the .equals("") usages with .isEmpty()

Changes: https://git.openjdk.java.net/jdk/pull/29/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=29&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252999
  Stats: 234 lines in 150 files changed: 0 ins; 0 del; 234 mod
  Patch: https://git.openjdk.java.net/jdk/pull/29.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/29/head:pull/29

PR: https://git.openjdk.java.net/jdk/pull/29