Do not give fatal error on report when no fields selected

This is a trivial fix to ensure that when a report that inherits the main 'select' function is rendered with no fields selected there is no hard error - by adding SELECT 1 instead of leaving the select clause missing. 

This is an upstream of a fix that has been in extended reports for some time. I don't know of an obvious way to replicate it but I feel there is no risk of harm here (reports that do weird things manage their own selects) and it helps people writing custom reports.