Bug 26784: Silence warns about comparing undefined values for MARC modification templates

Bug 26784: Silence warns about comparing undefined values for MARC modification templates

This patch simply checks if conditionals are defined before comparing

We don't need to check actions because you cannot save an action without an action :-)

To test:
1 - Define several actions for a MARC modification template
2 - Ensure you have actions with no conditionals
3 - Tail the intranet logs
4 - Browse to Tools->MARC modification templates
5 - Click 'Edit actions' for the template you setup above
6 - Note warns like:
    [2020/10/22 12:30:18] [WARN] Use of uninitialized value in string eq at /kohadevbox/koha/tools/marc_modification_templates.pl line 127.
7 - Apply patch
8 - Restart all the things
9 - Edit actions again and note no more warns

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>