settings: Make CAMO_KEY Optional.
settings: Make CAMO_KEY Optional.

This ensures that CAMO_KEY is always defined, so that mypy_django_plugin
will be able to identify its type.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip