[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/regressions/kt35668.fir.kt

@@ -0,0 +1,20 @@
+// !LANGUAGE: +NewInference
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+fun main() {
+    val baseDir: String? = ""
+    val networkParameters: String? = ""
+    if (baseDir != null) {
+        if (networkParameters != null) {
+            Unit
+        } else if (true){
+            return
+        } else {
+            return
+        }
+    } else {
+        return
+    }
+
+    networkParameters.length // unsafe call
+}

compiler/testData/diagnostics/tests/regressions/kt35668.kt

@@ -0,0 +1,20 @@
+// !LANGUAGE: +NewInference
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+fun main() {
+    val baseDir: String? = ""
+    val networkParameters: String? = ""
+    if (baseDir != null) {
+        if (networkParameters != null) {
+            Unit
+        } else if (true){
+            return
+        } else {
+            return
+        }
+    } else {
+        return
+    }
+
+    <!DEBUG_INFO_SMARTCAST!>networkParameters<!>.length // unsafe call
+}

compiler/testData/diagnostics/tests/regressions/kt35668.txt

@@ -0,0 +1,3 @@
+package
+
+public fun main(): kotlin.Unit