recent_topics: Don't process hotkeys in empty table.
recent_topics: Don't process hotkeys in empty table.

The fixes errors when we are trying process user input before
anything is initialized in recent topics.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip