Fix response with empty producer

cmd/streams/producer.go

@@ -29,7 +29,7 @@ func (p *Producer) GetMedias() []*streamer.Media {
 
 		var err error
 		p.element, err = GetProducer(p.url)
-		if err != nil {
+		if err != nil || p.element == nil {
 			log.Error().Err(err).Str("url", p.url).Msg("[streams] probe producer")
 			return nil
 		}

cmd/streams/stream.go

@@ -2,6 +2,7 @@ package streams
 
 import (
 	"encoding/json"
+	"errors"
 	"github.com/AlexxIT/go2rtc/pkg/streamer"
 )
 
@@ -78,7 +79,7 @@ func (s *Stream) AddConsumer(cons streamer.Consumer) (err error) {
 
 	// can't match tracks for consumer
 	if len(consumer.tracks) == 0 {
-		return nil
+		return errors.New("couldn't find the matching tracks")
 	}
 
 	s.consumers = append(s.consumers, consumer)