PYTHON-2113 An empty authSource URI option is not valid

Update spec test runner for PYTHON-1846.

Author: ShaneHarvey

pymongo/uri_parser.py

@@ -294,6 +294,9 @@ def split_options(opts, validate=True, warn=False, normalize=True):
 
     if validate:
         options = validate_options(options, warn)
+        if options.get('authsource') == '':
+            raise InvalidURI(
+                "the authSource database cannot be an empty string")
 
     if normalize:
         options = _normalize_options(options)