pgsql: Fix handling of NULL distances in KNN-GiST

Fix handling of NULL distances in KNN-GiST

In order to implement NULL LAST semantic GiST previously assumed distance to
the NULL value to be Inf.  However, our distance functions can return Inf and
NaN for non-null values.  In such cases, NULL LAST semantic appears to be
broken.  This commit fixes that by introducing separate array of null flags for
distances.

Backpatch to all supported versions.

Discussion: https://postgr.es/m/CAPpHfdsNvNdA0DBS%2BwMpFrgwT6C3-q50sFVGLSiuWnV3FqOJuQ%40mail.gmail.com
Author: Alexander Korotkov
Backpatch-through: 9.4

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e6af7b367cf46fc385307efcef5a1fece0e5fef3

Modified Files
--------------
src/backend/access/gist/gistget.c          | 70 ++++++++++++++++++++----------
src/backend/access/gist/gistscan.c         | 25 +++++++++--
src/backend/access/index/indexam.c         | 14 ++++--
src/backend/access/spgist/spgscan.c        |  1 +
src/include/access/genam.h                 |  4 +-
src/include/access/gist_private.h          | 26 +++++++++--
src/test/regress/expected/create_index.out |  2 +-
7 files changed, 106 insertions(+), 36 deletions(-)