argp: modified behaviour for empty group header

[Alex Regueiro]

The following patch for argp-help.c causes a blank line to be left between groups of options in `—help` output. This now occurs when an argp_option with the `doc` field a null-string is specified as a group header; previously no blank line was left between groups, even if such an argp_option element was specified (c.f. http://www.gnu.org/software/libc/manual/html_node/Argp-Option-Vectors.html#Argp-Option-Vectors). This would appear to be sensible behaviour, and certainly doesn’t detract from any usage case, as I see it. Incidentally, the behaviour for group headers where the `doc` field is non-empty is already sensible; it leaves a blank line in `—help` output, followed by the header text itself.	

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