pgsql: 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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/276393f53efbf08f72190221e9c1ef2a28e7fc66

Modified Files
--------------
src/backend/commands/copyfrom.c    |  9 ++++++++-
src/test/regress/expected/copy.out | 17 +++++++++++++++++
src/test/regress/sql/copy.sql      | 15 +++++++++++++++
3 files changed, 40 insertions(+), 1 deletion(-)