filter: return on null filter from open and close

So that we don't have to include the if(filter) open_filter(filter)
block everywhere, we introduce the guard in the function itself. This
should simplify quite a bit of code.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

filter.c

@@ -351,6 +351,8 @@ int cgit_open_filter(struct cgit_filter *filter, ...)
 {
 	int result;
 	va_list ap;
+	if (!filter)
+		return 0;
 	va_start(ap, filter);
 	result = filter->open(filter, ap);
 	va_end(ap);
@@ -359,6 +361,8 @@ int cgit_open_filter(struct cgit_filter *filter, ...)
 
 int cgit_close_filter(struct cgit_filter *filter)
 {
+	if (!filter)
+		return 0;
 	return filter->close(filter);
 }