Create from Usage: Fix generation of extensions with nullable receiver

#KT-23796 Fixed

idea/testData/quickfix/createFromUsage/createFunction/call/nullableReceiver.kt

@@ -0,0 +1,4 @@
+// "Create extension function 'String?.notExistingFun'" "true"
+fun context(p: String?) {
+    p.<caret>notExistingFun()
+}

idea/testData/quickfix/createFromUsage/createFunction/call/nullableReceiver.kt.after

@@ -0,0 +1,8 @@
+// "Create extension function 'String?.notExistingFun'" "true"
+fun context(p: String?) {
+    p.notExistingFun()
+}
+
+private fun String?.notExistingFun() {
+    TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
+}