rocketchat: Only set message content if it exists.
rocketchat: Only set message content if it exists.

Not sure where those come from since we discovered this with production
data.

Signed-off-by: Florian Pritz <753f544d2d01592750fb4bd29251b78abcfc8ecd@xinu.at>

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