Don't decode empty response body

Some neutron POST and PUT APIs don't return response body,
for instance, dhcp-agent-network-add and l3-agent-router-add.
This patch add a check for response body.

Change-Id: Ia772927a28cb83c4c88680b5add925b12978dc5d
Related-Bug: #1555921

neutronclient/tests/unit/test_cli20.py

@@ -793,6 +793,11 @@ class ClientV2TestJson(CLITestV20Base):
         self.mox.VerifyAll()
         self.mox.UnsetStubs()
 
+    def test_deserialize_without_data(self):
+        data = u''
+        result = self.client.deserialize(data, 200)
+        self.assertEqual(data, result)
+
 
 class CLITestV20ExceptionHandler(CLITestV20Base):
 

neutronclient/v2_0/client.py

@@ -305,7 +305,8 @@ class ClientBase(object):
 
     def deserialize(self, data, status_code):
         """Deserializes a JSON string into a dictionary."""
-        if status_code == 204:
+        # TODO(hichihara): Remove checking 204 in bug 1611167
+        if status_code == 204 or not data:
             return data
         return serializer.Serializer().deserialize(
             data)['body']