[FIR] Fix checking for null exhaustiveness with expression of Nothing? type

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirWhenExhaustivenessTransformer.kt

@@ -97,6 +97,10 @@ class FirWhenExhaustivenessTransformer(private val bodyResolveComponents: BodyRe
                         }
                     }
                     is FirQualifiedAccessExpression -> {
+                        if (argument.typeRef.isNullableNothing) {
+                            data.containsNull = true
+                            return
+                        }
                         val reference = argument.calleeReference as? FirResolvedNamedReference ?: return
                         val symbol = (reference.resolvedSymbol.fir as? FirEnumEntry)?.symbol ?: return
 
@@ -185,6 +189,12 @@ class FirWhenExhaustivenessTransformer(private val bodyResolveComponents: BodyRe
                     is FirResolvedQualifier -> {
                         argument.typeRef.accept(this, data)
                     }
+
+                    is FirQualifiedAccessExpression -> {
+                        if (argument.typeRef.isNullableNothing) {
+                            data.containsNull = true
+                        }
+                    }
                 }
             }
         }