audit: print empty EXECVE args

[ Upstream commit ea956d8be91edc702a98b7fe1f9463e7ca8c42ab ]

Empty executable arguments were being skipped when printing out the list
of arguments in an EXECVE record, making it appear they were somehow
lost.  Include empty arguments as an itemized empty string.

Reproducer:
	autrace /bin/ls "" "/etc"
	ausearch --start recent -m execve -i | grep EXECVE
	type=EXECVE msg=audit(10/03/2018 13:04:03.208:1391) : argc=3 a0=/bin/ls a2=/etc

With fix:
	type=EXECVE msg=audit(10/03/2018 21:51:38.290:194) : argc=3 a0=/bin/ls a1= a2=/etc
	type=EXECVE msg=audit(1538617898.290:194): argc=3 a0="/bin/ls" a1="" a2="/etc"

Passes audit-testsuite.  GH issue tracker at
https://github.com/linux-audit/audit-kernel/issues/99



Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
[PM: cleaned up the commit metadata]
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>