diff.c: remove useless check for value != NULL

diff.c: remove useless check for value != NULL

It is not necessary to check if value != NULL before calling
'parse_lldiff_command' as there is already a check inside this
function.

By the way this patch also improves the existing check inside
'parse_lldiff_command' by using:
	return config_error_nonbool(var);
instead of:
	return error("%s: lacks value", var);

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>