events: Add test to remove existing value for custom profile field.

Adds a test for when a value for a user's custom profile field is
removed and not set to a new value. The omission of this event in
the tests was noted as a possibility in #22103, which updated the
API documentation for these events having `null` for the field
value.

When adding the test discovered that the events logic was not
deleting the field from the user object and instead setting it to
`None`, so fixes that logic as well. There was a similar bug fixed
in commit 96c61a1a41 for when custom profile fields are removed
from a realm.

zerver/lib/event_schema.py

@@ -1068,7 +1068,7 @@ check_realm_user_remove = make_checker(realm_user_remove_event)
 custom_profile_field_type = DictType(
     required_keys=[
         ("id", int),
-        ("value", str),
+        ("value", OptionalType(str)),
     ],
     optional_keys=[
         ("rendered_value", str),