consider na_string() default for na.strings

[jmbarbone]

waiver function for explicit na.strings setting

na_string <- function() {
  structure("na_string", class = "openxlsx2_waiver")
}

is_na_string <- function(x) {
  is_waiver(x) && isTRUE(x == "na_string")
}

@JanMarvin , I did some testing and was able to implement this without having to update any of the tests. I'm pretty sure my poor handling of these is what's wrong with #471 .

This would help with using explicit values (definitely my preference; and we only have to check this once) but also (with another small internal adjustment) allow users to set their default strings value in options().

If we would want to use

foo <- function(
    ...
    na.strings = getOption("openxlsx2.na.strings", na_strings())
    ...
) {
  ...
}

We could allow zero length vectors (character()) to replace the NULL behavior within write_data2(); which currently errors through txt_to_is(character()). Explicitly setting na.strings = NULL would have the same effect; as would not setting it.