Do nothing when resource has no title

This fixes:

```
warning @ controller_websocket:142
throw:{badarg,[{erlang,iolist_to_binary,[[undefined," ",[]]],[]},
               {mod_rsc_name,calculate_name,4,
                             [{file,"priv/modules/mod_rsc_name/mod_rsc_name.erl"},
                              {line,59}]},
               {mod_rsc_name,observe_rsc_update,3,
                             [{file,"priv/modules/mod_rsc_name/mod_rsc_name.erl"},
                              {line,40}]},
               {m_rsc_update,'-update/4-fun-1-',7,
                             [{file,"src/models/m_rsc_update.erl"},
                              {line,273}]},
               {z_db,transaction,3,
                     [{file,"src/dbdrivers/postgresql/z_db.erl"},{line,83}]},
               {m_rsc_update,update,4,
                             [{file,"src/models/m_rsc_update.erl"},
                              {line,306}]},
               {mod_oembed,preview_create,3,
                           [{file,"modules/mod_oembed/mod_oembed.erl"},
                            {line,326}]},
               {mod_oembed,observe_rsc_update,3,
                           [{file,"modules/mod_oembed/mod_oembed.erl"},
                            {line,79}]}]}
```