Since Expression Language 3.0, it is valid to use += operator for string concatenation.
${(empty value)? "none" : value += " enabled"} // valid as of EL 3.0
Quoting EL 3.0 Specification.
String Concatenation Operator
To evaluate
A += B
- Coerce A and B to String.
- Return the concatenated string of A and B.
