RFR: 8299444 java.util.Set.copyOf allocates needlessly for empty input collections [v5]

[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.

Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:

  Adding comment clarifying where implicit nullchecks are made in Set and Map copyOf

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11847/files
  - new: https://git.openjdk.org/jdk/pull/11847/files/860b0c99..9d80411a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11847&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11847&range=03-04

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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