AT_BANNER() with empty argument

[Ralf Wildenhues]

Quick question: we document

 -- Macro: AT_BANNER (TEST-CATEGORY-NAME)
     This macro identifies the start of a category of related test
     groups.  When the resulting `testsuite' is invoked with more than
     one test group to run, its output will include a banner containing
     TEST-CATEGORY-NAME prior to any tests run from that category.  The
     banner should be no more than about 40 or 50 characters.  A blank
     banner will not print, effectively ending a category and letting
     subsequent test groups behave as though they are uncategorized
     when run in isolation.

but empty argument to AT_BANNER will not cause separation line from
previous category with current code, making it look as though the coming
test groups belong to the previous category.  Do you agree that we
should fix the code rather than the documentation?
(See 'Banners' test for an example of this.)

Asking because I'm working on removing line length limitations from
Autotest ($at_groups_all) and touching related code, so I figure might
as well fix the banners, too.

Thanks,
Ralf