fix: topic list crashes if no topics are available (#869)

This is due to the function expecting an object unpackable to 2
iterables, but an empty iterable is returned if no topics are available.
This happens only when using glob, which might have been the reason why
this bug was not caught earlier.

rosapi/src/rosapi/proxy.py

@@ -65,9 +65,13 @@ def get_topics_and_types(topics_glob):
 
         # Add topics with unknown type messages.
         unknown_type = topics.difference([x for x, _ in topic_types])
-        return zip(* topic_types + [[x,''] for x in unknown_type])
+        topic_types.extend([[x, ''] for x in unknown_type])
+        if topic_types:
+            return zip(*topic_types)
+        else:
+            return [], []
     except:
-        return []
+        return [], []
 
 
 def get_topics_for_type(type, topics_glob):