Feature Request: Add `null.ok = TRUE` Parameter to `checkmate::check_names`

[mnlang]

Description: I would like to request the addition of the null.ok = TRUE parameter to the checkmate::check_names function. This feature would allow for more concise and elegant input validation when checking if an object is either a list or NULL, and if it is a list, ensuring that it has specific names.

Current Limitation: Currently, for example, the checkmate::check_list function supports the null.ok parameter, allowing us to check if an object is either a list or NULL. However, the checkmate::check_names function does not support the null.ok parameter. This limitation makes it challenging to perform both checks in a single assert call.

Proposed Usage: With the proposed feature, the code could be simplified as follows:

foo <- function(x) {
  checkmate::assert(
    checkmate::check_list(x, null.ok = TRUE, names = "named"),
    checkmate::check_names(names(x), must.include = c("a", "b"), null.ok = TRUE) # NOTE: `null.ok` currently not a supported argument!
  )
  # ... 
}

Am I overlooking something, or what is your proposed usage? Thank you for considering this feature request.