fix(19-default-global-errors): set value from None to empty dictionary

This commit bears the change of setting the default value of global errors from None to an empty dictionary in the global configuration class.

closes 19

apimatic_core/configurations/global_configuration.py

@@ -26,7 +26,7 @@ def __init__(
             self, http_client_configuration=HttpClientConfiguration()
     ):
         self._http_client_configuration = http_client_configuration
-        self._global_errors = None
+        self._global_errors = {}
         self._global_headers = {}
         self._additional_headers = {}
         self._auth_managers = {}