`JsonPointer.empty()` should NOT indicate match of a property with key of ""

[cowtowncoder]

Currently the "empty" JsonPointer instance (obtained by JsonPointer.empty() or trying to compile empty String (zero-length)) is distinct from one that matches empty property name, /, regarding behavior.
Empty JsonPointer matches the current node whereas one with "empty property" would match value of JSON Object with empty String. Specifically, for JSON Object like:

{ "" : "text" }

1. JsonPointer.empty() would match JSON Object itself
2. JsonPointer.compile("") would match the String value text

However: currently (as of 2.13), empty JsonPointer:

• Returns '""' for getMatchingProperty()
• Retuns true for mayMatchProperty()
• But does NOT actually match with matchProperty("")

Of these, first 2 cases should be fixed.

It is understood that there may be backwards-compatibility challenges in changing the behavior: however, handling of the "empty" JsonPointer is important for proper handling of traversal.