Dialyzer fixes - Githubissues

Diplomat.Key.parent field was using the wrong type (Diplomat.Entity.t instead of Diplomat.Key.t)
Several fields on Diplomat.Key are optional and need type specs that allow for nil.
Some type specs were using %{} to represent a map type, but this literally means an empty map so they were replaced with the type map().
Replaced a few instances of pattern matching on %{} with when is_map(...).

peburrows / diplomat