FIR: don't emit SENSELESS_NULL_IN_WHEN when value is always null

This case (value is always null) contradicts the error message which says
"Expression under 'when' is never equal to null".

compiler/testData/diagnostics/tests/when/whenOnNothing.fir.kt

@@ -3,7 +3,7 @@
 // exhaustive
 fun test1(n: Nothing) = when (n) { }
 fun test2(n: Nothing?) = when (n) {
-    <!SENSELESS_NULL_IN_WHEN!>null<!> -> {}
+    <!SENSELESS_COMPARISON!>null<!> -> {}
 }
 
 // not exhaustive