bookmark nick: Treat empty string as no nick

And prevent the JID() call from failing with InvalidJid because of the
empty resource.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

poezio/core/commands.py

@@ -526,7 +526,7 @@ class CommandCore:
 
         # Validate / Normalize
         try:
-            if nick is None:
+            if not nick:
                 jid = JID(room)
             else:
                 jid = JID('{}/{}'.format(room, nick))