Fix the return value for a NULL key

An example where the filter functions can be called with a NULL key is
when a bluetooth discovery fails to retrieve the name of the remote
device. In such case, we have no information to detect whether the
bluetooth device matches a known dive computer or not, and thus it
shouldn't be filtered out.

src/descriptor.c

@@ -495,7 +495,7 @@ static int
 dc_filter_internal (const void *key, const void *values, size_t count, size_t size, dc_match_t match, void *params_dst, const void *params_src, size_t params_size)
 {
 	if (key == NULL)
-		return 0;
+		return 1;
 
 	for (size_t i = 0; i < count; ++i) {
 		if (match (key, (const unsigned char *) values + i * size)) {