sql: don't run any query if there are no evolution to load

wcs/sql.py

@@ -893,6 +893,8 @@ class SqlFormData(SqlMixin, wcs.formdata.FormData):
         # on a serie of formdata and in that case, it's more efficient to
         # optimize the process loading all evolutions in a single batch query.
         object_dict = dict([(x.id, x) for x in values if x.id and x._evolution is None])
+        if not object_dict:
+            return
         conn, cur = get_connection_and_cursor()
         sql_statement = '''SELECT id, who, status, time, comment, parts, formdata_id
                              FROM %s_evolutions''' % cls._table_name