[1.10.x] Refs #27186 -- Fixed model form default fallback for CheckboxSelectMultiple.

Backport of 87c5e7efeb from master

django/forms/widgets.py

@@ -812,6 +812,11 @@ class CheckboxSelectMultiple(RendererMixin, SelectMultiple):
         # require all checkboxes to be checked instead of at least one.
         return False
 
+    def value_omitted_from_data(self, data, files, name):
+        # HTML checkboxes don't appear in POST data if not checked, so it's
+        # never known if the value is actually omitted.
+        return False
+
 
 class MultiWidget(Widget):
     """