LP#1745462: avoid trying to validate empty fields

LP#1745462: avoid trying to validate empty fields

This patch is the belt to the previous patch's suspenders and ensures
that the web staff client does not attempt to authority-validate headings
fields in the MARC editor if they're empty (i.e., no subfield values).

To test
-------
[1] In the web staff client, create a new bib. Ensure that at least one of the
    authority-controlled fields has no subfield values.
[2] Hit the Validate button.
[3] Note that the field(s) with empty headings show the tick
    mark indicating that they've been "validated".
[4] Apply the patch and repeat steps 1 and 2. This time, empty headings
    field should not be checked.
[5] Verify that validating headings that are not empty does continue
    to work.

Patch inspired by Mike Rylander.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>