Setting a column to `NULL` in a schema should check for its existence

[hfrick]

The "reference in new issue" feature seems to not work right now 🦄 but here's the issue following on from this comment
https://github.com/rstudio/pointblank/pull/642/files#r2293299337

Essentially, it'd be great if specifying b = NULL in a schema would mean a check on existence without the type check. Currently, you can skip the type check but that could also pass because the column is missing.

library(pointblank)

# baseline: passes
data.frame(a = 1:2) |>
  col_schema_match(col_schema(a = "integer"))
#>   a
#> 1 1
#> 2 2

# add b to data frame and to schema as NULL and strict check fails as it should
data.frame(a = 1:2, b = 1:2) |> 
  col_schema_match(col_schema(a = "integer", b = NULL))
#> Error: Failure to validate that column schemas match.
#> The `col_schema_match()` validation failed beyond the absolute threshold level (1).
#> * failure level (1) >= failure threshold (1)

# relaxing `is_exact` allows the check to pass
data.frame(a = 1:2, b = 1:2) |>
  col_schema_match(col_schema(a = "integer", b = NULL), is_exact = FALSE)
#>   a b
#> 1 1 1
#> 2 2 2

# but it still passes when b is missing from the data frame
# i.e. it's not a check for existence
data.frame(a = 1:2) |>
  col_schema_match(col_schema(a = "integer", b = NULL), is_exact = FALSE)
#>   a
#> 1 1
#> 2 2

^{Created on 2025-08-26 with reprex v2.1.1}