[NI] Soften restictions on using Nothing as proper constraint for full call completion

Consider lower `Nothing` constraint non-proper only if there is a dependant not analyzed postponed atom.
Early completion to `Nothing` provides data flow info for smart casts.

KT-35668 Fixed

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

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