pgsql: Fast default trigger and expand_tuple fixes

Fast default trigger and expand_tuple fixes

Ensure that triggers get properly filled in tuples for the OLD value.
Also fix the logic of detecting missing null values. The previous logic
failed to detect a missing null column before the first missing column
with a default. Fixing this has simplified the logic a bit.

Regression tests are added to test changes. This should ensure better
coverage of expand_tuple().

Original bug reports, and some code and test scripts from Tomas Vondra

Backpatch to release 11.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7636e5c60fea83a9f3cd2ad278c0819b98941c74

Modified Files
--------------
src/backend/access/common/heaptuple.c | 51 ++++----
src/backend/commands/trigger.c | 5 +-
src/test/regress/expected/fast_default.out | 191 ++++++++++++++++++++++++++++-
src/test/regress/sql/fast_default.sql | 114 ++++++++++++++++-
4 files changed, 328 insertions(+), 33 deletions(-)