argp: modified behaviour for empty group header

[Alex Regueiro]

I followed all the rules I could see and understand on the libc web pages... it's possible I've missed something, but things should be in order.

> On 8 Jan 2015, at 00:14, Roland McGrath <roland@hack.frob.com> wrote:
> 
>> Absolutely, but a blank (note: not NULL) line should be respected,
>> instead of ignored, I'd think. There's no reason to include them unless
>> they do something, really. And this additional behaviour would seem
>> useful to me.
> 
> OK, I see the point.  Have you followed the contribution checklist?