Create from Usage: Fix generation of extensions with nullable receiver

#KT-23796 Fixed

idea/testData/quickfix/createFromUsage/createVariable/property/nullableReceiver.kt

@@ -0,0 +1,6 @@
+// "Create extension property 'String?.notExistingVal'" "true"
+fun foo(n: Int) {}
+
+fun context(p: String?) {
+    foo(p.<caret>notExistingVal)
+}