[1.11.x] Fixed #29273 -- Prevented initial selection of empty choice in multiple choice widgets.

Regression in b52c7300.

Backport of f3b69f97 from master.

django/forms/widgets.py

@@ -648,6 +648,8 @@ class ChoiceWidget(Widget):
 
     def format_value(self, value):
         """Return selected values as a list."""
+        if value is None and self.allow_multiple_selected:
+            return []
         if not isinstance(value, (tuple, list)):
             value = [value]
         return [force_text(v) if v is not None else '' for v in value]

docs/releases/1.11.12.txt

@@ -12,3 +12,7 @@ Bugfixes
 * Fixed a regression in Django 1.11.8 where combining two annotated
   ``values_list()`` querysets with ``union()``, ``difference()``, or
   ``intersection()`` crashed due to mismatching columns (:ticket:`29229`).
+
+* Fixed a regression in Django 1.11 where an empty choice could be initially
+  selected for the ``SelectMultiple`` and ``CheckboxSelectMultiple`` widgets
+  (:ticket:`29273`)

tests/forms_tests/widget_tests/test_checkboxselectmultiple.py

@@ -32,10 +32,12 @@ class CheckboxSelectMultipleTest(WidgetTest):
 
     def test_render_none(self):
         """
-        If the value is None, none of the options are selected.
+        If the value is None, none of the options are selected, even if the
+        choices have an empty option.
         """
-        self.check_html(self.widget(choices=self.beatles), 'beatles', None, html=(
+        self.check_html(self.widget(choices=(('', 'Unknown'),) + self.beatles), 'beatles', None, html=(
             """<ul>
+            <li><label><input type="checkbox" name="beatles" value="" /> Unknown</label></li>
             <li><label><input type="checkbox" name="beatles" value="J" /> John</label></li>
             <li><label><input type="checkbox" name="beatles" value="P" /> Paul</label></li>
             <li><label><input type="checkbox" name="beatles" value="G" /> George</label></li>

tests/forms_tests/widget_tests/test_selectmultiple.py

@@ -9,7 +9,7 @@ class SelectMultipleTest(WidgetTest):
 
     def test_format_value(self):
         widget = self.widget(choices=self.numeric_choices)
-        self.assertEqual(widget.format_value(None), [''])
+        self.assertEqual(widget.format_value(None), [])
         self.assertEqual(widget.format_value(''), [''])
         self.assertEqual(widget.format_value([3, 0, 1]), ['3', '0', '1'])
 
@@ -35,10 +35,12 @@ class SelectMultipleTest(WidgetTest):
 
     def test_render_none(self):
         """
-        If the value is None, none of the options are selected.
+        If the value is None, none of the options are selected, even if the
+        choices have an empty option.
         """
-        self.check_html(self.widget(choices=self.beatles), 'beatles', None, html=(
-            """<select multiple="multiple" name="beatles">
+        self.check_html(self.widget(choices=(('', 'Unknown'),) + self.beatles), 'beatles', None, html=(
+            """<select multiple name="beatles">
+            <option value="">Unknown</option>
             <option value="J">John</option>
             <option value="P">Paul</option>
             <option value="G">George</option>