I thought that invoking the command without an argument toggled it between off and on, but apparently that's only when it's invoked interactively.
Passing a nil argument to a minor mode function call now ENABLES
the minor mode unconditionally. This is so that you can write e.g.
(add-hook 'text-mode-hook #'foo-mode)
to enable foo-mode in Text mode buffers, removing the need for
'turn-on-foo-mode' style functions. This affects all mode commands
defined by 'define-minor-mode'. If called interactively, the mode
command still toggles the minor mode.