The Subquery class which Exists inherits from defaulted to using single-valued
querie's field if no output_field was explicitly specified on initialization
which was bypassing the Exists.output_field defined at the class level.
Moving Subquery's dynamic output_field resolution to _resolve_output_field
should make sure the fallback logic is only performed if required.
Regression in 08654a99bb.
Thanks Oli Warner for the detailed report.
Mattia Procopio
Artem Tabolin
dzhuang
Simon Charette
Tim Graham