cubicweb #2544358 jsonexport view should be usable on empty rsets [done]

jsonexport view is not selectable if resultset is empty. Therefore, instead of returning [], it returns the standard HTML boilerplate that tells "This view cannot be used to display this data".

This breaks jsonp clients expecting json data.

done in3.15.8
load left0.000
closed by#459d0c48dfaf [test] fix possible views test (closes #2544358), #3ae893f9ec84 [json] enable jsonexport selection on empty rsets (closes #2544358)