[FIR] Allow Int? === null

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

^KT-62646

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

@@ -103,6 +103,7 @@ object FirEqualityCompatibilityChecker : FirEqualityOperatorCallChecker(MppCheck
         val oneIsNotNull = !l.type.isNullable || !r.type.isNullable
 
         return when {
+            l.type.isNullableNothing || r.type.isNullableNothing -> Applicability.APPLICABLE
             l.isIdentityLess || r.isIdentityLess -> Applicability.INAPPLICABLE_AS_IDENTITY_LESS
             oneIsNotNull && shouldReportAsPerRules1(l, r, context) -> getInapplicabilityFor(l, r)
             else -> Applicability.APPLICABLE