settings: Type JITSI_SERVER_URL as being Optional.
settings: Type JITSI_SERVER_URL as being Optional.

We fixed the case when handling `JITSI_SERVER_URL` being `None`, but the
type annotation didn't get updated along with the fix
https://github.com/zulip/zulip/commit/2f9d4f5a96746f05791d6d45d8545a3cc0855866

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

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