narrow: Cut redundant conditional in getIfNoMessages, and add a type.
narrow: Cut redundant conditional in getIfNoMessages, and add a type.

Like its type says: `messages` will always be an Array,
and therefore truthy.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile