[NI] Discriminate Nothing? result type from nullability constraints

Nullability constraints should not be chosen when proper
argument constraints for variables in contravariant position present.

^KT-32106 Fixed
^KT-33166 Fixed

compiler/tests-spec/testData/diagnostics/notLinked/dfa/pos/4.kt

@@ -285,7 +285,7 @@ fun case_24(a: ((() -> Unit) -> Unit), b: (() -> Unit)) {
 }
 
 // TESTCASE NUMBER: 25
-fun case_25(a: (() -> Unit) -> Unit, b: (() -> Unit) -> Unit = if (<!SENSELESS_COMPARISON!>a == null<!>) <!DEBUG_INFO_EXPRESSION_TYPE("(() -> kotlin.Unit) -> kotlin.Unit & kotlin.Nothing"), DEBUG_INFO_SMARTCAST!>a<!> else <!TYPE_MISMATCH!>{{}}<!>) {
+fun case_25(a: (() -> Unit) -> Unit, b: (() -> Unit) -> Unit = if (<!SENSELESS_COMPARISON!>a == null<!>) <!DEBUG_INFO_EXPRESSION_TYPE("(() -> kotlin.Unit) -> kotlin.Unit & kotlin.Nothing")!>a<!> else {{}}) {
     <!DEBUG_INFO_EXPRESSION_TYPE("(() -> kotlin.Unit) -> kotlin.Unit")!>a<!>
     <!DEBUG_INFO_EXPRESSION_TYPE("(() -> kotlin.Unit) -> kotlin.Unit")!>b<!>
 }