Bug 11319: [QA follow-up] Remove a warning from GetModificationTemplates

Bug 11319: [QA follow-up] Remove a warning from GetModificationTemplates

Removes this warning: Use of uninitialized value $template_id in string eq
at C4/MarcModificationTemplates.pm line 84.

GetModificationTemplates has no template_id if called from
marc_modification_templates.pl without operation (first click from
interface) and from tools/stage-marc-import.pl.

Slightly adjusted the POD lines accordingly.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>