streamdata: omitempty category field

This is treated as optional when handling the prompt.

Signed-off-by: Yonah <contrib@gensokyo.uk>

streamdata.go

@@ -125,7 +125,7 @@ type VOD struct {
 	// Day of stream start.
 	Date time.Time `json:"date"`
 	// Free-form category string.
-	Category string `json:"category"`
+	Category string `json:"category,omitempty"`
 }
 
 // ChannelMismatchError describes a mismatching [Ident.Channel] passed to [Channel.Add].