RichTextLabel (RTL): add_text(), append_text(), add_image() (and probably others) do not work on orphan RTL

### Tested versions

Reproducible in: v4.4.dev7.official [46c8f8c5c], v4.3.stable.official [77dcf97d8]

### System information

Godot v4.4.dev7 - Windows 11 (build 22631) - Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce MX450 (NVIDIA; 32.0.15.5613) - 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz (8 threads)

### Issue description

add_text(), append_text(), add_image() and probably other similar functions do not work on orphan RichTextLabel. Setting the text property directly works.

In the Minimal reproduction project, I only used append_text(), but in my current project in v4.3.stable.official add_image() doesn't work either.

I think it's a bug because I didn't find anything that explains why it behaves like that.
It's pretty simple to bypass putting add_child() before add_*(). Deferring the add_* functions works too. However, I think it's not the expected behavior.

### Steps to reproduce

- Create a RichTextLabel by code (instantiating a scene or creating a new RTL with enough size)
- Add some text using add_text() or append_text()
- Add the RTL to the tree with add_child()

Can be done on a brand new project. I added a Minimal reproduction project anyway.

### Minimal reproduction project (MRP)

[orphan_RTL_test.zip](https://github.com/user-attachments/files/18232032/orphan_RTL_test.zip)

Start with the attached orphan_RTL_test.zip minimal reproduction and reproduce the behavior on an orphan RichTextLabel before it is added to the tree. Compare append_text(), add_text(), and add_image() with direct text assignment; done means these methods behave as expected when called before add_child().

Written by the indexing model from the issue text.