Empty configuration_schema raises InvalidTwine

[thclark]

Parsing the following Configuration Schema raises an InvalidTwine:

"configuration_schema": {},

It seems to me that for general usability, this should be allowable and simply treated as not present.

Note that an empty python dict tests as boolean false:

dict1 = {}
if dict1:
    print("dict1 Not Empty")
else:
    print("dict1 is Empty")
>>> dict1 is Empty

So we either have to explicitly handle the case using that truth test, or have to update the schema to allow for an empty dict.