### 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)

} else {
		if (p_item->subitems.size() && p_item->type != ITEM_TABLE) {
			return p_item->subitems.front()->get();
		} else if (p_item->type == ITEM_FRAME) {
			return nullptr;
		} else if (p_item->E->next()) {
			return p_item->E->next()->get();
		} else {
			
			while (p_item->type != ITEM_FRAME && !p_item->E->next()) {
				p_item = p_item->parent;
			}
			if (p_item->type != ITEM_FRAME) {
				return p_item->E->next()->get();
			} else {
				return nullptr;
			}
		}
	}
}
RichTextLabel::Item *RichTextLabel::_get_prev_item(Item *p_item, bool p_free) const {
	if (p_free) {
		if (p_item->subitems.size()) {
			return p_item->subitems.back()->get();
		} else if (!p_item->parent) {
			return nullptr;
		} else if (p_item->E->prev()) {
			return p_item->E->prev()->get();
		} else {