Only evaluate default values as required when doing COPY FROM

Only evaluate default values as required when doing COPY FROM

Commit 9f8377f7a2 was a little too eager in fetching default values.
Normally this would not matter, but if the default value is not valid
for the type (e.g. a varchar that's too long) it caused an unnecessary
error.

Complaint and fix from Laurenz Albe

Backpatch to release 16.

Discussion: https://postgr.es/m/75a7b7483aeb331aa017328d606d568fc715b90d.camel@cybertec.at