The Javadoc of MethodTree.isOverriding() states that the method returns
— true if overriding, null if it cannot be decided (method symbol not resolved or lack of byte code for super types), false if not overriding.

However, the current implementation returns misleading results when the hierarchy of the class containing the method contains unknowns. In such cases, null should be returned, because it is not possible to reliably determine if the method is an override or not.

In addition, we should also make sure that the different rules using this method correctly deal with the null value.