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.

prioritynormal
typebug
done in3.15.8
load0.100
load left0.000
closed by#459d0c48dfaf [test] fix possible views test (closes #2544358), #3ae893f9ec84 [json] enable jsonexport selection on empty rsets (closes #2544358)