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

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

```release-note
NONE
```

api4/channel.go

@@ -1447,7 +1447,7 @@ func addChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	postRootId, ok := props["post_root_id"].(string)
-	if ok && len(postRootId) != 0 && !model.IsValidId(postRootId) {
+	if ok && postRootId != "" && !model.IsValidId(postRootId) {
 		c.SetInvalidParam("post_root_id")
 		return
 	}