forms: Fix to_python method signature.

According to the documentation, `to_python` should account for `None`
being a possible value to be processed.

See also: https://docs.djangoproject.com/en/4.0/howto/custom-model-fields/#converting-values-to-python-objects.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zerver/forms.py

@@ -511,7 +511,7 @@ class AuthenticationTokenForm(TwoFactorAuthenticationTokenForm):
 
 
 class MultiEmailField(forms.Field):
-    def to_python(self, emails: str) -> List[str]:
+    def to_python(self, emails: Optional[str]) -> List[str]:
         """Normalize data to a list of strings."""
         if not emails:
             return []