Fixed #231 -- Avoid fetching entire queryset from db

Patched ciritcal memory leak in ModelSelect2 widgets.
Boolean evaluation of queryset makes django fetch all
elements.

Thanks to @magicjohnson for discovering this issue

CHANGELOG.md

@@ -1,6 +1,9 @@
 Changelog Summary
 =================
 
+### v5.4.1
+* Fixed memory leak in `ModelSelect2Mixin` and subclasses
+
 ### v5.4.0
 * Added `Select2TagWidget` a light widget with tagging support
 

django_select2/__init__.py

@@ -9,4 +9,4 @@ The app includes Select2 driven Django Widgets and Form Fields.
 
 """
 
-__version__ = "5.4.0"
+__version__ = "5.4.1"

django_select2/forms.py

@@ -351,7 +351,7 @@ class ModelSelect2Mixin(object):
         :return: Filtered queryset
         :rtype: :class:`.django.db.models.QuerySet`
         """
-        if not queryset:
+        if queryset is None:
             queryset = self.get_queryset()
         search_fields = self.get_search_fields()
         select = Q()