Looks up a value in a VariantDict.
key is not found in
dictionary, null is returned.
expected_type string specifies what type of value is expected. If the value associated with
key has a different
type then null is returned.
If the key is found and the value has the correct type, it is returned. If
expected_type was specified then any non-
null return value will have this type.
the key to look up in the dictionary
a VariantType, or null
the value of the dictionary key, or null