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/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirEqualityCompatibilityChecker.kt

@@ -131,8 +131,9 @@ object FirEqualityCompatibilityChecker : FirEqualityOperatorCallChecker() {
             }
         }
         // We only report `SENSELESS_NULL_IN_WHEN` if `lType = type` because `lType` is the type of the when subject. This diagnostic is
-        // only intended for cases where the branch condition contains a null.
-        if (expression.source?.elementType != KtNodeTypes.BINARY_EXPRESSION && type === lType) {
+        // only intended for cases where the branch condition contains a null. Also, the error message for SENSELESS_NULL_IN_WHEN
+        // says the value is *never* equal to null, so we can't report it if the value is *always* equal to null.
+        if (expression.source?.elementType != KtNodeTypes.BINARY_EXPRESSION && type === lType && !compareResult) {
             reporter.reportOn(expression.source, FirErrors.SENSELESS_NULL_IN_WHEN, context)
         } else {
             reporter.reportOn(expression.source, FirErrors.SENSELESS_COMPARISON, expression, compareResult, context)

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

compiler/tests-spec/testData/diagnostics/linked/expressions/when-expression/p-6/pos/5.2.fir.kt

@@ -218,7 +218,7 @@ fun case_23(value_1: Nothing) {
 // TESTCASE NUMBER: 24
 fun case_24(value_1: Nothing?) = when (value_1) {
     <!SENSELESS_COMPARISON!>throw Exception()<!>, <!SENSELESS_COMPARISON!><!RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY!>return<!> ""<!> -> ""
-    <!SENSELESS_NULL_IN_WHEN!>null<!>, <!RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY!>return<!> <!RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY!>return<!> <!RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY!>return<!> "", throw throw throw Exception() -> ""
+    <!SENSELESS_COMPARISON!>null<!>, <!RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY!>return<!> <!RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY!>return<!> <!RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY!>return<!> "", throw throw throw Exception() -> ""
     else -> ""
 }
 

compiler/tests-spec/testData/diagnostics/notLinked/dfa/pos/37.fir.kt

@@ -199,7 +199,7 @@ fun case_17(x: Boolean?, y: Boolean?) {
                     else -> if (true) if (true) if (true) if (true) if (true) x!! else x!! else x!! else x!! else x!! else x!!
                 }
                 <!SENSELESS_COMPARISON!>false<!> -> x!!
-                <!SENSELESS_NULL_IN_WHEN!>null<!> -> if (true) if (true) if (true) if (true) if (true) x!! else x!! else x!! else x!! else x!! else x!!
+                <!SENSELESS_COMPARISON!>null<!> -> if (true) if (true) if (true) if (true) if (true) x!! else x!! else x!! else x!! else x!! else x!!
             } else x!! else x!! else x!! else x!! else x!!
         }
         break@loop

compiler/tests-spec/testData/diagnostics/notLinked/dfa/pos/62.fir.kt

@@ -154,7 +154,7 @@ fun case_10(x: Any): String {
 fun case_11(x: Any?): String? {
     if (x is Nothing?) {
         return when(x) {
-            <!SENSELESS_NULL_IN_WHEN!>null<!> -> null
+            <!SENSELESS_COMPARISON!>null<!> -> null
         }
     }
     return ""
@@ -168,7 +168,7 @@ fun case_11(x: Any?): String? {
 fun case_12(x: Any?): String? {
     if (x == null) {
         return when(x) {
-            <!SENSELESS_NULL_IN_WHEN!>null<!> -> null
+            <!SENSELESS_COMPARISON!>null<!> -> null
         }
     }
     return ""
@@ -182,7 +182,7 @@ fun case_12(x: Any?): String? {
 fun case_13(x: Any?): String? {
     if (x === null) {
         return when(x) {
-            <!SENSELESS_NULL_IN_WHEN!>null<!> -> null
+            <!SENSELESS_COMPARISON!>null<!> -> null
         }
     }
     return ""
@@ -196,7 +196,7 @@ fun case_13(x: Any?): String? {
 fun case_14(x: Any?): String? {
     x as Nothing?
     return when(x) {
-        <!SENSELESS_NULL_IN_WHEN!>null<!> -> null
+        <!SENSELESS_COMPARISON!>null<!> -> null
     }
 }