Converts a NULL to NA; or returns the argument unchanged otherwise
This is useful for missing list elements (which returns NULL), but when the missing element in refClass should be NA (and NULL would return an error)
NULL_to_NA(var_or_NULL)
var_or_NULL
: A variable of any kind or NULLNULL_to_NA(NA) ## NA NULL_to_NA(NULL) ## NA NULL_to_NA("hi") ## "hi"