message_events: Support undefined stream_name.
message_events: Support undefined stream_name.

When moving a topic within a stream that is deactivated, the stream
may not be present in stream_data.  Avoid throwing an exception in
this situation by leaving stream_name as undefined; existing logic
seems to anticipate that as a possibility, so we don't need a broader
change.

Longer term, we may want to just send to clients basic data about
archived streams that the user has access to.

Fixes: #17271

rht/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,kou/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,kou/zulip,andersk/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,zulip/zulip,kou/zulip,andersk/zulip,andersk/zulip