Feature: Allow for the group and user functions to not require a write value.

Feature: Allow for the group and user functions to not require a write value.

Several of these functions can be used to just check the validity of an ID or name.
Use the NULL pointer case to support this rather than writing a completely new functions for this logic.

For example, the `f_account_group_id_by_name()` function can have NULL for the `id` parameter.
In this case, nothing is written for the `id` and the function does not error out on invalid parameter (for when `id` is NULL).
The same logic is applied for the `f_account_group_name_by_id()` for the `name` parameter.

The same logic is applied for both user functions.

The account by functions are not updated as this is not needed.
Update the documentation for the account by functions, documenting that NULL is not allowed.