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/[email protected]

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/910eb61b2bbdb729c242ed20d5f6e0a392eb62da

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(-)