LP#1373578: fix translatability of hold placement errors

LP#1373578: fix translatability of hold placement errors

This patch fixes a circumstance where ->maketext() could be
passed an empty string, causing it to emit the PO file header text
rather than a desired string.

To test
-------
[1] Configure Evergreen to use a TPAC localization (e.g.,
    the Czech translation).
[2] Enable the "Has Local Copy Alert" library setting.
[3] In the OPAC, change locale and attempt to place a
    hold that would trigger the alert saying that there's
    already a local copy available.
[4] Note that the alert reads as "Project-Id-Version: evergreen
    Report-Msgid-Bugs-To: ..."
[5] Apply the patch.
[6] Try step 3; this time the alert will be translated correctly,
    e.g., "Ve vaší domovské knihovně je již dostupný exemplář."

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Ben Shum <ben@evergreener.net>