Copy: Fix exception on copying KtFile to non-KtFile

#KT-22669 Fixed

idea/testData/refactoring/copy/copyKtFileToTextFile/after/foo/test.kt

@@ -0,0 +1,8 @@
+package foo
+
+val a = 42
+
+// comment
+
+// some other comment
+val s = ""

idea/testData/refactoring/copy/copyKtFileToTextFile/after/foo/test.kt.txt

@@ -0,0 +1,8 @@
+package foo
+
+val a = 42
+
+// comment
+
+// some other comment
+val s = ""