Both of these queries show unhelpful errors instead of a helpful warning message like "no data was found". Also, looking at the code, there seems to be no way for the visualizer to show a custom error message in the error band above visualizer.
Returns a single empty object
#defaultView:Map SELECT ?place ?placeLabel ?location WHERE {}
{ "head" : { "vars" : [ "place", "placeLabel", "location" ] }, "results" : { "bindings" : [ { } ] } }
Returns empty response
#defaultView:Map SELECT ?place ?placeLabel ?location WHERE { wd:Q0 wdt:P0 ?location . }
{ "head" : { "vars" : [ "place", "placeLabel", "location" ] }, "results" : { "bindings" : [ ] } }