parameters: export empty BSON document even if there are no parameters to save

- otherwise a 'param reset_all' won't be saved until a parameter is changed

src/lib/parameters/parameters.cpp

@@ -1371,11 +1371,6 @@ static int param_verify(int fd)
 int
 param_export(int fd, param_filter_func filter)
 {
-	/* no modified parameters -> we are done */
-	if (param_values == nullptr) {
-		return 0;
-	}
-
 	int result = -1;
 	perf_begin(param_export_perf);
 
@@ -1410,6 +1405,12 @@ param_export(int fd, param_filter_func filter)
 		goto out;
 	}
 
+	// no modified parameters, export empty BSON document
+	if (param_values == nullptr) {
+		result = 0;
+		goto out;
+	}
+
 	while ((s = (struct param_wbuf_s *)utarray_next(param_values, s)) != nullptr) {
 		if (filter && !filter(s->param)) {
 			continue;