argp: modified behaviour for empty group header

[Alex Regueiro]

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.

> On 8 Jan 2015, at 00:09, Roland McGrath <roland@hack.frob.com> wrote:
> 
> I think the current behavior is what's intended.  When there is no group
> header line, the purpose of groups is to change the order of options
> in --help output.