Fix yet more problems with incorrectly-constructed zero-length arrays.

Fix yet more problems with incorrectly-constructed zero-length arrays.

Commit 716ea626a attempted to fix the problem of building 1-D zero-size
arrays once and for all.  But it turns out that contrib/intarray has some
code that doesn't use construct_array() but just builds arrays by hand,
so it didn't get the memo.  This appears to affect all of subarray(),
intset_subtract(), inner_int_union(), inner_int_inter(), and
intarray_concat_arrays().

Back-patch into v11.  In the past we've not back-patched this type of
change, but since v11 is still in beta it seems all right to include
this fix in it.  Besides it's more consistent to make the fix in v11
where 716ea626a appeared.

Report and patch by Alexey Kryuchkov, some cosmetic adjustments by me

Report: https://postgr.es/m/153053285112.13258.434620894305716755@wrigleys.postgresql.org
Discussion: https://postgr.es/m/CAN85JcYphDLYt4CpMDLZjjNVqGDrFJ5eS3YF=wLAhFoDQuBsyg@mail.gmail.com