Make KtPureElement.getParent() nullable #KT-19760 Fixed

The reason is KtFile.getParent() is a containing directory which is nullable

compiler/frontend/src/org/jetbrains/kotlin/psi/KtPureElement.java

@@ -34,7 +34,6 @@ public interface KtPureElement {
     /**
      * Returns parent source element.
      */
-    @NotNull
     PsiElement getParent();
 
     @NotNull

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/LabelResolver.kt

@@ -90,7 +90,6 @@ object LabelResolver {
 
         if (parent is KtValueArgument) {
             // f ({}) or f(p = {}) or f (fun () {})
-            // NB: parent of KtElement is really nullable!!!
             val argList = parent.parent ?: return null
             val call = argList.parent
             if (call is KtCallExpression) {