push_notifications: Fix events with no message_ids.
push_notifications: Fix events with no message_ids.

This fixes our triggering a RabbitMQ event to send a push notification
to remove the empty set of push notifications, resulting from not
using the correct data structure to determine which message IDs to look at.

This was causing a lot of visible exceptions when running
the `test_messages.py` test suite.

andersk/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,showell/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,kou/zulip,kou/zulip,synicalsyntax/zulip,eeshangarg/zulip,synicalsyntax/zulip,synicalsyntax/zulip,punchagan/zulip,showell/zulip,brainwane/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,kou/zulip,synicalsyntax/zulip,brainwane/zulip,rht/zulip,zulip/zulip,andersk/zulip,synicalsyntax/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,brainwane/zulip,rht/zulip,kou/zulip,brainwane/zulip,andersk/zulip,showell/zulip,showell/zulip,showell/zulip,eeshangarg/zulip,synicalsyntax/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,brainwane/zulip,punchagan/zulip,kou/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,showell/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,zulip/zulip,brainwane/zulip,zulip/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,showell/zulip