format_missing() updates a rounding_specification object so that
missing values are printed as the user specifies.
format_missing(rspec, replace_na_with)a rounding_specification object (see round_spec).
a character value that replaces missing values.
an object of class rounding_specification.
rspec <- round_spec()
rspec <- format_missing(rspec, 'oh no!')
table_value(x = c(pi, NA), rspec)
#> [1] "3.1" "oh no!"