KT-531 Support nullable iterator in for()

compiler/testData/codegen/controlStructures/forNullableIntArray.jet

@@ -12,5 +12,10 @@ fun box() : String {
     System.out?.println(sum)
     if(sum != 10) return "b failed"
 
+    val d : Array<Int?>? = null
+    for (el in d) {
+        sum = sum + (el ?: 7)
+    }
+
     return "OK"
 }

compiler/testData/codegen/controlStructures/forPrimitiveIntArray.jet

@@ -10,5 +10,10 @@ fun box() : String {
     }
     if(sum != 10) return "a failed"
 
+    val b : IntArray? = null
+    for (el in b) {
+        sum = sum + el
+    }
+
     return "OK"
 }

compiler/testData/codegen/controlStructures/forUserType.jet

@@ -66,6 +66,14 @@ fun box() : String {
     System.out?.println(sum)
     if(sum != 10) return "b failed"
 
+    val c7 = MyCollection5()
+    sum = 0
+    for (el in c7) {
+        sum = sum + el
+    }
+    if(sum != 0) return "c7 failed"
+
+
     return "OK"
 }
 
@@ -115,3 +123,14 @@ class MyCollection4() {
         fun hasNext() = k > 0
     }
 }
+
+class MyCollection5() {
+    fun iterator() : MyIterator? = null
+
+    class MyIterator() {
+        var k : Int = 5
+
+        fun next() : Int = k--
+        fun hasNext() = k > 0
+    }
+}