Fix #1809: All-masked integer columns are written as <TD/>, even though
that violates the VOTable standard prior to version 1.3
Band-aid cyclical import problems that prevent running packages that
define Table types on their own in the test framework.
Add test for out-of-range null value
Fix regression test for io.votable.validate
Add CHANGES entry
Merge pull request #1809 from mdboom/vo/empty-fields

How to use ``astropy.io.votable`` to fix non standard-compliant tables?