RFR: 8267529: StringJoiner can create a String that breaks String::equals [v2]

[Claes Redestad]

> This patch fixes a regression caused by https://bugs.openjdk.java.net/browse/JDK-8265237 where the result of String.join always get a UTF-16 coder if the delimiter is UTF-16, even when no delimiter is emitted.

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Add join of zero strings, clarify comment

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4228/files
  - new: https://git.openjdk.java.net/jdk/pull/4228/files/0e2ca7cd..b4b9d14d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4228&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4228&range=00-01

  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4228.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4228/head:pull/4228

PR: https://git.openjdk.java.net/jdk/pull/4228