mypy: Fix argument type for get_active_streams.
mypy: Fix argument type for get_active_streams.

We now make it mandatory to pass in the Realm object.

If this function was ever called with None, I am scared
to know what the expected results were at the time of
writing.

eeshangarg/zulip,andersk/zulip,andersk/zulip,kou/zulip,kou/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,zulip/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,kou/zulip,kou/zulip,rht/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip