t6000: fix a careless test library add-on.

It tried to "restore" GIT_AUTHOR_EMAIL environment variable but
the variable started out as unset, so ended up setting it to an
empty string.  This is now caught as an error.

Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t6000lib.sh

@@ -51,7 +51,12 @@ as_author()
 
 	export GIT_AUTHOR_EMAIL="$_author"
 	"$@"
-        export GIT_AUTHOR_EMAIL="$_save"
+	if test -z "$_save"
+	then
+		unset GIT_AUTHOR_EMAIL
+	else
+		export GIT_AUTHOR_EMAIL="$_save"
+	fi
 }
 
 commit_date()