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)
+}

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

@@ -0,0 +1,11 @@
+// "Create extension property 'String?.notExistingVal'" "true"
+fun foo(n: Int) {}
+
+fun context(p: String?) {
+    foo(p.notExistingVal)
+}
+
+private val String?.notExistingVal: Int
+    get() {
+        TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
+    }