MongoDB: fix for empty username/password (#7487)

Author: yoshiokatsuneo

redash/query_runner/mongodb.py

@@ -215,10 +215,10 @@ def _get_db(self):
             if readPreference:
                 kwargs["readPreference"] = readPreference
 
-        if "username" in self.configuration:
+        if self.configuration.get("username"):
             kwargs["username"] = self.configuration["username"]
 
-        if "password" in self.configuration:
+        if self.configuration.get("password"):
             kwargs["password"] = self.configuration["password"]
 
         db_connection = pymongo.MongoClient(self.configuration["connectionString"], **kwargs)