add: use advise_if_enabled for ADVICE_ADD_EMPTY_PATHSPEC

add: use advise_if_enabled for ADVICE_ADD_EMPTY_PATHSPEC

Since 93b0d86aaf (git-add: error out when given no arguments.,
2006-12-20) we display a message when no arguments are given to "git
add".

Part of that message was converted to advice in bf66db37f1 (add: use
advise function to display hints, 2020-01-07).

Following the same line of reasoning as in the previous commit, it is
sensible to use advise_if_enabled() here.

Therefore, use advise_if_enabled() in builtin/add.c to show the
ADVICE_ADD_EMPTY_PATHSPEC advice, and don't bother checking there the
visibility of the advice or displaying the instruction on how to disable
it.

Also add a test for these messages, in order to detect a possible
change in them.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>