Support DEDUCTION of empty subtypes

[drekbour]

Support deduction of empty subtypes (classes with no distinguishing properties at all) against empty json ({}).

Incoming null, <absent> and use of ignoreUnknownProperties will not NOT match an empty subtype. In keeping with the affirmative matching used by the rest of the deduction mode, there can be only a single empty subtype and only an explicitly empty object ({}) will match it.

Inspired by #3137 - thanks to @xJoeWoo for the report

This is raised against 2.12 but can be rebased onto 2.13 on request

[cowtowncoder]

Ok, so my instinct is this should be safe enough for 2.12 -- the only possible concern is the change to check current token is not END_OBJECT... but I think I'll take my chances with that. So this will be available in 2.12.4 if that gets released before 2.13.

[drekbour]

The END_OBJECT conditional is because all preceding property-based type deserialisers had to parse at least one property to work. Only with this PR do we allow {} through the normal path, where it arrives already placed on END_OBJECT token. At least that was my reading of this method when making the change.

[cowtowncoder]

Makes sense -- did not mean to imply there is no reason (and similar checks have been sometimes needed for various reasons in other places). Just that it is possible that some edge case somewhere might be relying on specific set of things (f.ex, XML handling), and might break either legitimately or (more likely) due to incorrect assumptions.

Having said that check looks sensible to me so I'm ok in assuming it all works unless proven otherwise :)