Answer NULL at the distance sentinel the catalog states through distance_sentinel

[estebanzimanyi]

tools/codegen_duck_udfs.py reads the guard if (datum_eq(result, distance_sentinel(basetype),
basetype)), on which a PG wrapper returns NULL for a distance computed in the base type, as the
test r == std::numeric_limits<decltype(r)>::max() on the result in the type MEOS returns, before
its conversion to the declared float, as emit_temporal_box tests the sentinel of a box. The
nearest approach distance between two temporal numbers, and between a temporal number and a
number, is NULL where the two share no time, over tint, tbigint and tfloat.

Why: distance_sentinel answers the maximum of the type the distance is expressed in where no
distance is computed, and NAD_tnumber_tnumber and NAD_tnumber_number return NULL on it; the
catalog carries that guard in shape.nullableResult, while SCALAR_NULL_GUARD read only the
result == DBL_MAX form, so the six nearestApproachDistance functions over numbers returned the
sentinel itself as a distance.

Witness: Gen_nad_tfloat_tfloat returns (double) r with no test, r being what nad_tfloat_tfloat
answers, so for two tfloat values sharing no time the row holds DBL_MAX, and over tint and
tbigint the maxima of int and int64 widened to a double; with this change the row is NULL.

Measured: over the catalog the shared provision derives from MobilityDB master 4c4457d2ec, the
generator writes the same 1972 functions and 7389 registrations, and the six
Gen_nad_{tint_int, tint_tint, tbigint_bigint, tbigint_tbigint, tfloat_float, tfloat_tfloat}
take ExecuteWithNulls and the test; no other function differs. The committed surface carries the
six as the generator writes them; generated_temporal_udfs_2.cpp compiles warning-free.
test/sql/parity/036_tnumber_distance.test states the three NULLs.