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>

diff.c

@@ -166,13 +166,8 @@ int git_diff_ui_config(const char *var, const char *value)
 	if (!prefixcmp(var, "diff.")) {
 		const char *ep = strrchr(var, '.');
 
-		if (ep != var + 4) {
-			if (!strcmp(ep, ".command")) {
-				if (!value)
-					return config_error_nonbool(var);
-				return parse_lldiff_command(var, ep, value);
-			}
-		}
+		if (ep != var + 4 && !strcmp(ep, ".command"))
+			return parse_lldiff_command(var, ep, value);
 	}
 
 	return git_diff_basic_config(var, value);