[FIR] Source for implicit iterator calls in for should not be null

^KT-62111 fixed


Merge-request: KT-MR-13029
Merged-by: Egor Kulikov <Egor.Kulikov@jetbrains.com>

compiler/testData/diagnostics/tests/incompleteCode/checkNothingIsSubtype.fir.kt

@@ -10,6 +10,6 @@ fun test(nothing: Nothing?) {
 }
 
 fun sum(a : IntArray) : Int {
-for (n
-<!SYNTAX!>return<!><!SYNTAX!><!> "?"
+<!ITERATOR_MISSING!>for (n
+<!SYNTAX!>return<!><!SYNTAX!><!> "?"<!>
 <!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>