NoValueOfOnStringType: keep String.valueOf when the argument can be null

[Niloyyy]

• Fixes Recipe NoValueOfOnStringType incorrectly removes String.valueOf on null-initialized constants, introducing runtime NPE #1009

The problem

String.valueOf(s) equals s only when s is not null — for a null String it produces "null". The recipe removed the call for any String-typed argument that wasn't a method invocation, so a nullable variable lost its conversion:

private static final String INCIDENT_OPEN_TASK = null;

templateBody.replace("$INCIDENT_OPEN_TASK", String.valueOf(INCIDENT_OPEN_TASK));
// became
templateBody.replace("$INCIDENT_OPEN_TASK", INCIDENT_OPEN_TASK);

Running both:

with valueOf : x=null
without      : THREW java.lang.NullPointerException

Method invocations were already excluded, and the reason is the same one that applies here — the value may be null. Identifiers, field accesses and casts are no safer. This also lines up the behavior with the recipe's own description, which says the simplification applies "when the argument to String#valueOf(arg) is a string literal".

The change

For String arguments, require the expression to be demonstrably non-null — a non-null literal, or a string concatenation.

The one thing I wanted to avoid was over-correcting. Inside a concatenation the removal is safe even for nulls, because + already renders a null operand as "null":

"p" + String.valueOf(f)   ->  pnull
"p" + f                   ->  pnull

My first attempt blocked that too, which would have lost a legitimate simplification. So removeValueOfForStringConcatenation now accepts String arguments alongside primitives, and that case keeps working.

Tests

• Three added, tagged to Recipe NoValueOfOnStringType incorrectly removes String.valueOf on null-initialized constants, introducing runtime NPE #1009: the reported constant, nullable variables in return/assignment/argument positions, and a positive test that concatenation is still simplified. All 13 existing tests are unchanged and still pass.

Full suite locally on JDK 21: 2277 tests, 0 failures.