Adjust code to PG14 reltuples changes

[svenklemm]

PG14 changes the initial value of pg_class.reltuples to -1 to allow
differentiating between an empty relation and a relation where
ANALYZE has not yet run.

postgres/postgres@3d351d916b

[codecov]

Codecov Report

Merging #3370 (4d42d6a) into master (001f5d9) will decrease coverage by 1.57%.
The diff coverage is 88.45%.

@@            Coverage Diff             @@
##           master    #3370      +/-   ##
==========================================
- Coverage   92.17%   90.59%   -1.58%     
==========================================
  Files         110      212     +102     
  Lines       14177    35621   +21444     
==========================================
+ Hits        13067    32272   +19205     
- Misses       1110     3349    +2239     

Impacted Files	Coverage Δ
src/bgw/launcher_interface.c	72.22% <ø> (+1.63%)	⬆️
src/catalog.h	100.00% <ø> (ø)
src/chunk.c	95.90% <ø> (+1.37%)	⬆️
src/chunk.h	100.00% <ø> (ø)
src/chunk_adaptive.c	92.57% <ø> (+2.78%)	⬆️
src/chunk_append/chunk_append.c	97.41% <ø> (+0.90%)	⬆️
src/chunk_append/exec.c	95.84% <ø> (+1.66%)	⬆️
src/chunk_append/explain.c	82.81% <ø> (ø)
src/chunk_append/planner.c	93.38% <ø> (-2.27%)	⬇️
src/chunk_append/transform.c	96.66% <ø> (-3.34%)	⬇️
... and 280 more

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[mkindahl]

LGTM