[NI] Fix completion for ILT when Nothing constraint is present

Full completion should not be done if lower `Nothing`
is the only proper constraint when constraint with ILT type is present.
ILT will be selected as a resulting type and transformed into `Int`
without attention to possible restrictions from outer calls.

compiler/testData/diagnostics/tests/when/kt9929.kt

@@ -1,5 +1,5 @@
 // !WITH_NEW_INFERENCE
-val test: Int = <!NI;TYPE_MISMATCH, NI;TYPE_MISMATCH!>if (true) {
+val test: Int = <!NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, NI;TYPE_MISMATCH!>if (true) {
     when (2) {
         1 -> 1
         else -> <!OI;NULL_FOR_NONNULL_TYPE!>null<!>