[FE 1.0] Process builder inference stub types with different nullability properly

^KT-49887 Fixed

compiler/testData/codegen/box/inference/builderInference/kt49887.kt

@@ -0,0 +1,12 @@
+// WITH_STDLIB
+
+fun test(): Int = buildList {
+    add(1)
+    add(2)
+
+    val number = removeLastOrNull() ?: throw Exception()
+}.singleOrNull() ?: throw Exception()
+
+fun box(): String {
+    return if (test() == 1) "OK" else "NOK"
+}