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 08654a99bbdd09049d682ae57cc94241534b29f0.
Thanks Oli Warner for the detailed report.
Backport of 277ed072094ad87fc6b2c4669f21d43b1f39043c from master
0fd21febe7