MM-31062: Rewrite empty string checks to be more idiomatic (#16587)

https://mattermost.atlassian.net/browse/MM-31062

```release-note
NONE
```

model/outgoing_webhook.go

@@ -140,7 +140,7 @@ func (o *OutgoingWebhook) IsValid() *AppError {
 		return NewAppError("OutgoingWebhook.IsValid", "model.outgoing_hook.is_valid.user_id.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	if len(o.ChannelId) != 0 && !IsValidId(o.ChannelId) {
+	if o.ChannelId != "" && !IsValidId(o.ChannelId) {
 		return NewAppError("OutgoingWebhook.IsValid", "model.outgoing_hook.is_valid.channel_id.app_error", nil, "", http.StatusBadRequest)
 	}