don't encode empty streams

- separate emptyBuffer method

internals/overlord/logstate/loki.go

@@ -71,15 +71,14 @@ func (c *lokiClient) Flush(ctx context.Context) error {
 		return nil // no-op
 	}
 
-	defer func() {
-		for svc := range c.entries {
-			c.entries[svc] = c.entries[svc][:0]
-		}
-	}()
+	defer c.emptyBuffer()
 
 	// Build request
 	req := lokiRequest{}
 	for service, entries := range c.entries {
+		if len(entries) == 0 {
+			continue
+		}
 		stream := lokiStream{
 			Labels: map[string]string{
 				"pebble_service": service,
@@ -120,6 +119,12 @@ func (c *lokiClient) Flush(ctx context.Context) error {
 	return nil
 }
 
+func (c *lokiClient) emptyBuffer() {
+	for svc := range c.entries {
+		c.entries[svc] = c.entries[svc][:0]
+	}
+}
+
 type lokiRequest struct {
 	Streams []lokiStream `json:"streams"`
 }