### Godot version

v4.0.2.stable.official [7a0977ce2]

### System information

Windows 10, not platform specific

### Issue description

Methods `get_character_line` and `get_character_paragraph` return wrong indices for characters that are first in the respective line/paragraph. See the attached screenshot.

![image](https://user-images.githubusercontent.com/892180/236546781-692603be-eb09-419e-822e-d4152a66d9b6.png)


### Steps to reproduce

Run attached project.

The script goes through all characters in `RichTextLabel` and prints the respective character, line index, and paragraph index. None of them should be -1 since we are iterating through valid indices.

### Minimal reproduction project

[RichTextLabel.zip](https://github.com/godotengine/godot/files/11409399/RichTextLabel.zip)

selection.active = false;
	queue_redraw();
}
void RichTextLabel::selection_copy() {
	String txt = get_selected_text();
	if (!txt.is_empty()) {
		DisplayServer::get_singleton()->clipboard_set(txt);
	}
}
void RichTextLabel::select_all() {
	if (!selection.enabled) {
		return;
	}
	Item *it = main;
	Item *from_item = nullptr;
	Item *to_item = nullptr;
	while (it) {
		if (it->type != ITEM_FRAME) {
			if (!from_item) {
				from_item = it;
			} else {
				to_item = it;
			}