[FIR] Allow Int? === null

It was decided to allow this comparison
duting the last DM.

^KT-62646

compiler/testData/diagnostics/testsWithStdLib/equalityCompatibilityCommonCases.fir.kt

@@ -87,7 +87,7 @@ fun incompatibleIdentityRegardlessNullabilitySmartCast(a: Any?, b: Any?) {
 
 fun incompatibleIdentityRegardlessNullabilityWithNull(a: Int?) {
     a == null
-    <!FORBIDDEN_IDENTITY_EQUALS_WARNING!>a === null<!>
+    a === null
 }
 
 fun incompatibleIdentityRegardlessNullabilityWithValueClasses(c: C?, d: D?) {