Avoid array allocation when concatenating with empty string

Currently, when concatenation of two strings is evaluated and one of them happens to be an empty string, another is copied completely, including the backing array (see java.lang.StringConcatHelper::simpleConcat). While creating a new String instance is required in this case by JLS 15.18.1, it's still allowed to reuse the internal array reducing the allocations and CPU usage.

A simple benchmark demonstrates the possible improvement:

@Param({"", "longlonglongline"})
  String data;

  @Param({"", "longlonglongline"})
  String data2;

  @Benchmark
  public String plus() {
    return data + data2;
  }

Without patch it takes about 15ns to concatenate two empty strings and about 20ns to concatenate an empty string with "longlonglongline". After the patch is applied, the average time goes down to 6.6-7.0 ns.