JLS 15.18.1. "String Concatenation Operator +" unconditionally requires to create a new String object during concatenation with + (except for constant expressions).

This seems excessive when one or both operands are empty strings.
For example, given:
var s = "abc"
var t = ""
the expression
s + t == s
evaluates to false because s + t cannot just evaluate to s.

If nothing speaks against, concatenation with + should be allowed to return one of the operand if the other is empty.
This already happens with String.concat(String).