tests: when no return-type expectation

tests/fn.assertSignature.allNullable.phpt

@@ -11,6 +11,7 @@ $fn = function(?T1 $t1, ?T2 $t2): ?T3 {return new T3;};
 
 // Correct signature assertion
 assertSignature($fn, params(nullable(T1::class), nullable(T2::class)), nullable(T3::class));
+assertSignature($fn, params(nullable(T1::class), nullable(T2::class)), NULL); // no expectation about return type
 
 // parameter is nullable, non-nullable is expected
 Assert::exception(