Schema resolver avoid nullpointerexception

[nestabur]

When in the inheritance tree there is a definition without properties but extending another specifying the type "object", a null pointer exception is thrown due to the fact that the swagger model returns null when getting the properties (instead of an empty map)

[coveralls]

Coverage increased (+0.2%) to 89.612% when pulling 7b3221d on channelsuite:schema-resolver-avoid-nullpointerexception into ba1594c on RobWin:master.