mqtt: fix unexpected behaviour

Currently mqtt_response with the unexpected key set to true fails if no
message is received. This is strange, was we don't expect a message, and
should only fail if a message is received.

Formatted stage:
  mqtt_publish:
    payload: 10.10.10.10
    topic: inet6/add
  mqtt_response:
    payload: !anything ''
    timeout: 5
    topic: vallumd/will
    unexpected: true

Errors:
E   tavern.util.exceptions.TestFailError: Test 'add IPv4 IP to IPv6 ipset' failed:
    - Expected '<Tavern YAML sentinel for anything>' on topic 'vallumd/will' but no such message received

Fix this by adding an extra check for the expected key when we received
no message.

Signed-off-by: Stijn Tintel <[email protected]>

tavern/_plugins/mqtt/response.py

@@ -181,11 +181,12 @@ def addwarning(w, *args, **kwargs):
 
             self._maybe_run_validate_functions(msg)
         else:
-            self._adderr(
-                "Expected '%s' on topic '%s' but no such message received",
-                expected_payload,
-                topic,
-            )
+            if not self.expected.get("unexpected"):
+                self._adderr(
+                    "Expected '%s' on topic '%s' but no such message received",
+                    expected_payload,
+                    topic,
+                )
 
         if self.errors:
             if warnings:
@@ -198,6 +199,9 @@ def addwarning(w, *args, **kwargs):
 
         saved = {}
 
+        if not msg:
+            return saved
+
         saved.update(self.maybe_get_save_values_from_save_block("json", msg.payload))
 
         saved.update(self.maybe_get_save_values_from_ext(msg, self.expected))