Prevent literal "None" placeholders (#591)

django_select2/forms.py

@@ -80,7 +80,7 @@ class Select2Mixin:
             default_attrs['data-allow-clear'] = 'false'
         else:
             default_attrs['data-allow-clear'] = 'true'
-            default_attrs['data-placeholder'] = self.empty_label
+            default_attrs['data-placeholder'] = self.empty_label or ""
 
         default_attrs.update(base_attrs)
         attrs = super().build_attrs(default_attrs, extra_attrs=extra_attrs)

tests/test_forms.py

@@ -95,7 +95,9 @@ class TestSelect2Mixin:
         multiple_select = self.multiple_form.fields['featured_artists']
         assert multiple_select.required is False
         assert multiple_select.widget.allow_multiple_selected
-        assert '<option value=""></option>' not in multiple_select.widget.render('featured_artists', None)
+        output = multiple_select.widget.render('featured_artists', None)
+        assert '<option value=""></option>' not in output
+        assert 'data-placeholder=""' in output
 
     def test_i18n(self):
         translation.activate('de')