Allow empty project list on organization team form (#10947)

This allows the field/widget in the form to be empty on submit.

- Fixes #10934

readthedocs/organizations/forms.py

@@ -208,6 +208,7 @@ def __init__(self, *args, **kwargs):
         self.fields["projects"] = forms.ModelMultipleChoiceField(
             queryset=self.organization.projects,
             widget=forms.CheckboxSelectMultiple,
+            required=False,
         )