[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/declarationChecks/destructuringDeclarations/ForLoopMissingLoopParameter.fir.kt

@@ -1,14 +1,14 @@
 fun useDeclaredVariables() {
-    for ((a, b)<!SYNTAX!><!>) {
+    <!ITERATOR_MISSING!>for ((a, b)<!SYNTAX!><!>) {
         a
         b
-    }
+    }<!>
 }
 
 fun checkersShouldRun() {
-    for ((@A a, _)<!SYNTAX!><!>) {
+    <!ITERATOR_MISSING!>for ((@A a, _)<!SYNTAX!><!>) {
 
-    }
+    }<!>
 }
 
 annotation class A