Fix change email error

Users without a `slack` field could not change their email address due
to the fact that doing so would attempt to set the slack column to null
(and nulls are not allowed).

This updates the save of the change email field to only modify the slack
value if it exists.

Issue #2784 Changing email on non-staff accounts creates non-null
            constraint violation

hypha/apply/users/forms.py

@@ -105,7 +105,8 @@ class EmailChangePasswordForm(forms.Form):
 
     def save(self, updated_email, name, slack, commit=True):
         self.user.full_name = name
-        self.user.slack = slack
+        if slack is not None:
+            self.user.slack = slack
         if commit:
             self.user.save()
         return self.user

hypha/apply/users/tests/test_forms.py

 from django.forms.models import model_to_dict
 from django.test import TestCase
 
-from ..forms import ProfileForm
+from ..forms import EmailChangePasswordForm, ProfileForm
 from .factories import StaffFactory, UserFactory
 
 
@@ -89,3 +89,18 @@ class TestStaffProfileForm(BaseTestProfileForm):
 
         self.staff.refresh_from_db()
         self.assertEqual(self.staff.slack, slack_name)
+
+
+class TestEmailChangePasswordForm(TestCase):
+    def setUp(self):
+        self.user = UserFactory()
+
+    def test_doesnt_error_on_null_slack_field(self):
+        form = EmailChangePasswordForm(self.user)
+        form.save('', '', None)
+
+    def test_can_update_slack(self):
+        slack_name = 'foobar'
+        form = EmailChangePasswordForm(self.user)
+        form.save('', '', slack_name)
+        self.assertEqual(self.user.slack, slack_name)