Stop accepting foreign_values => undef/rowobj in the resolver

Stop accepting foreign_values => undef/rowobj in the resolver

There are just a few spots that need this, things are complex enough as it is

Introduces a subtle change in behavior - now results of $foreign->get_columns
are scrutinized just as a plain hashref, and as a result the sanity checks are
somewhat relaxed.

There should not be any fallout due to this - tested on a wide range of
downstreams

Adjust some tested-for exceptions added in 7e5a0e7c as a result of the above

Read under -w