Public event filter: Make "all" option clearer (Z#23169843) (#4585)

* Public event filter: Make "all" option clearer

* Fix widget tests

* Update src/tests/presale/test_widget.py

Co-authored-by: Richard Schreiber <schreiber@rami.io>

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/presale/forms/organizer.py

@@ -21,6 +21,7 @@
 #
 from django import forms
 from django.conf import settings
+from django.utils.translation import pgettext
 from i18nfield.strings import LazyI18nString
 
 from pretix.base.models import EventMetaValue, SubEventMetaValue
@@ -66,7 +67,7 @@ def meta_filtersets(organizer, event=None):
             ).values_list("value", flat=True).distinct())
             choices = [(k, k) for k in sorted(existing_values)]
 
-        choices.insert(0, ("", ""))
+        choices.insert(0, ("", "-- %s --" % pgettext("filter_empty", "all")))
         if len(choices) > 1:
             fields[f"attr[{prop.name}]"] = {
                 "label": str(prop.public_label) or prop.name,