Improve Phoenix.Presence.get_by_key/2 documentation, return value, and typespec.

[jaspervanbrian]

Issue

#5759 states that issue #4514 wasn't addressed properly. Checking the docs for Phoenix.Presence.get_by_key/2, it does have inconsistencies between its return value and the typespec it's being referred to.

Improvements and Changes

Going by the given problem and proposed solution from #4514, I have made the changes:

• Improve and fix the presence typespec that returns a map with the :metas.
• If no presence found under the supplied key in socket_or_topic scope, return nil. Pros of having a nil return value instead of [] is that it's easier to evaluate not being a truthy value.
• Change the documented return value of Phoenix.Presence.get_by_key/2 to return a map instead of a list that contains the metas information, along with some additional info that might be returned on the Phoenix.Presence.fetch/2 callback.
• Change test specs on test/phoenix/presence_test.exs to check with is_nil instead of [] if the returned value on untracked key is empty.