Solve some warnings with the glossary suffixes

[amieiro]

Problem

We are getting some warnings with the glossary suffixes:

PHP Warning:  Undefined array key "abbreviation" in wp-content/plugins/GlotPress/gp-templates/helper-functions.php on line 409
PHP Warning:  foreach() argument must be of type array|object, null given in wp-content/plugins/GlotPress/gp-templates/helper-functions.php on line 409

Solution

This PR:

• removes all elements in the $suffixes array with empty values.
• skip computing the $suffixes[ $type ] with empty values.

Testing Instructions

• Translate some strings with the develop version and see some warnings in the debug.log file.
• Checkout to the branch of this PR. Now you should not see the previous warnings.

[trymebytes]

Tested, looks good to me!