Use NULL instead of 0 for 'arg' argument in dshash_create() calls.
A couple of dshash_create() callers provide 0 for the 'void *arg'
argument, which might give readers the incorrect impression that
this is some sort of "flags" parameter.
Reviewed-by: Andy Fan
Discussion: https://postgr.es/m/
20240119215941.GA1322079%40nathanxps13