Improve input checking for direct scoring metrics

[nikosbosse]

For example, in pit_sample() there is currently this:

  if (missing("true_values") | missing("predictions")) {
    stop("`true_values` or `predictions` missing in function 'pit_sample()'")
  }
  check_not_null(true_values = true_values, predictions = predictions)

This could be done in one function and applied to all other direct scoring methods as well.