Fix response obj string repr in case of no data

For example an Connection::authenticate() response does no contain a
data. This leads to an exception when working with the connection from
interactive python console.

Fixes #139.

Author: denis-ignatenko

tarantool/response.py

@@ -238,7 +238,7 @@ def __str__(self):
                 }
             }, sort_keys = True, indent = 4)
         output = []
-        for tpl in self._data:
+        for tpl in self._data or ():
             output.extend(("- ", repr(tpl), "\n"))
         if len(output) > 0:
             output.pop()