hg: jdk/sandbox: 8247605: Avoid array allocation when concatenating with empty string

[chris.hegarty at oracle.com]

Changeset: 20d92fe3ac52
Author:    tvaleev
Date:      2020-06-16 03:16 +0000
URL:       https://hg.openjdk.java.net/jdk/sandbox/rev/20d92fe3ac52

8247605: Avoid array allocation when concatenating with empty string
Reviewed-by: redestad, plevart

! src/java.base/share/classes/java/lang/StringConcatHelper.java
! test/micro/org/openjdk/bench/java/lang/StringConcat.java