Fix empty filters to match all items

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/zabbix/methods.go

@@ -319,6 +319,10 @@ func (ds *Zabbix) GetHosts(ctx context.Context, groupFilter string, hostFilter s
 }
 
 func filterHostsByQuery(items []Host, filter string) ([]Host, error) {
+	if filter == "" {
+		return items, nil
+	}
+
 	re, err := parseFilter(filter)
 	if err != nil {
 		return nil, err
@@ -354,6 +358,10 @@ func (ds *Zabbix) GetGroups(ctx context.Context, groupFilter string) ([]Group, e
 }
 
 func filterGroupsByQuery(items []Group, filter string) ([]Group, error) {
+	if filter == "" {
+		return items, nil
+	}
+
 	re, err := parseFilter(filter)
 	if err != nil {
 		return nil, err