RFR: 8299444 java.util.Set.copyOf allocates needlessly for empty input collections

[Viktor Klang]

Currently Set.copyOf allocates both a HashSet and a new empty array when the input collection is empty.

This patch avoids allocating anything for the case where the parameter collection's isEmpty returns true.

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Commit messages:
 - 8299444: java.util.Set.copyOf allocates needlessly for empty input collections

Changes: https://git.openjdk.org/jdk/pull/11847/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11847&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299444
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11847.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11847/head:pull/11847

PR: https://git.openjdk.org/jdk/pull/11847