colexec: fixed nullableArgs projection operators

Previously, the `Concat` projection operators on arrays directly copy nulls into
the output without performing the actual projection whenever either argument was
null. This is incorrect because `Concat` operator can actually handle null arguments,
and we want the actual projection result instead. Since currently the only
projection operator that needs `nullableArgs == true` behaviour is
`ConcatDatumDatum`, this PR fixed this by adding a boolean field `nullableArgs`
to the logic for operators with `Datum`, `Datum`. If we later introduce another
projection operation that has nullableArgs == true, we should update this code
accordingly.

Fixes: #83091

Release note (bug fix): Fixed a bug in `Concat` projection operators on arrays
that gave output of nulls when the projection operator can actually handle null
arguments and may result in a non-null output.