mysql: prevent the Pin from attaching empty tags

Author: talwai

ddtrace/contrib/mysql/patch.py

@@ -33,7 +33,7 @@ def _connect(func, instance, args, kwargs):
 
 def patch_conn(conn):
 
-    tags = {t: getattr(conn, a, '') for t, a in CONN_ATTR_BY_TAG.items()}
+    tags = {t: getattr(conn, a) for t, a in CONN_ATTR_BY_TAG.items() if getattr(conn, a, '') != ''}
     pin = Pin(service="mysql", app="mysql", app_type="db", tags=tags)
 
     # grab the metadata from the conn