datadog/mysql: when getting the size of the view db, it returns none …

…and gets a TypeError. This defaults to 0 if null instead of none

mysql/datadog_checks/mysql/mysql.py

@@ -1347,7 +1347,7 @@ def _query_size_per_schema(self, db):
 
         sql_query_schema_size = """
         SELECT   table_schema,
-                 SUM(data_length+index_length)/1024/1024 AS total_mb
+                 IFNULL(SUM(data_length+index_length)/1024/1024,0) AS total_mb
                  FROM     information_schema.tables
                  GROUP BY table_schema;
         """