Summary
-------
Fix `ExecutableElement.getReceiverType` to follow its specification and to return an object rather than null for "no type" results.
Problem
-------
In cases where the specification of `ExecutableElement.getReceiverType` required a sentinel object to be returned -- a `NoType` [type mirror](https://docs.oracle.com/en/java/javase/12/docs/api/java.compiler/javax/lang/model/type/NoType.html), with kind `NONE` -- the javac implementation returned `null`.
Solution
--------
Fix the implementation to follow the specification.
Specification
-------------
No specification change; behavioral change only.