Empty default for ARG_USE_ENV help text incorrect

[hemna]

The help text output for an ARG_USE_ENV is incorrect when the default is empty or isn't specified.

# ARG_USE_ENV([OS_CLOUD], [], [my blah help text])

or

# ARG_USE_ENV([OS_CLOUD], , [my blah help text])

Both of those result in

Environment variables that are supported:
	OS_CLOUD: my blah help text.)

I would expect it to be

Environment variables that are supported:
	OS_CLOUD: my blah help text.