RFR: 8325730: StringBuilder.toString allocation for the empty String [v2]

[Claes Redestad]

> JDK-8282429 accidentally removed an optimization (JDK-8240094) that ensured StringBuilder/StringBuffer::toString returns `""` when the builders are empty.
> 
> 
> Name                         Cnt     Base      Error   Test   Error   Unit  Change
> StringBuffers.emptyToString    5   12,289 ±    0,384  9,883 ± 0,721  ns/op   1,24x (p = 0,000*)
>   :gc.alloc.rate                 1862,398 ±   57,647  0,007 ± 0,000 MB/sec   0,00x (p = 0,000*)
>   :gc.alloc.rate.norm              24,000 ±    0,000  0,000 ± 0,000   B/op   0,00x (p = 0,000*)
>   :gc.count                        31,000             0,000         counts
>   :gc.time                         21,000                               ms
> StringBuilders.emptyToString   5    4,146 ±    0,567  0,646 ± 0,003  ns/op   6,42x (p = 0,000*)
>   :gc.alloc.rate                 9208,656 ± 1234,399  0,007 ± 0,000 MB/sec   0,00x (p = 0,000*)
>   :gc.alloc.rate.norm              40,000 ±    0,000  0,000 ± 0,000   B/op   0,00x (p = 0,000*)
>   :gc.count                        96,000             0,000         counts
>   :gc.time                         64,000                               ms
>   * = significant

Claes Redestad has updated the pull request incrementally with two additional commits since the last revision:

 - Revert StringBuffers removals
 - Update from review comments by @shipilev

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/17931/files
  - new: https://git.openjdk.org/jdk/pull/17931/files/7f9566b8..40cca3e3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17931&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17931&range=00-01

  Stats: 41 lines in 2 files changed: 38 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17931.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17931/head:pull/17931

PR: https://git.openjdk.org/jdk/pull/17931