### 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)

Item *item = p_item;
	while (item) {
		if (item->type == ITEM_HINT) {
			ItemHint *hint = static_cast<ItemHint *>(item);
			if (r_description) {
				*r_description = hint->description;
			}
			return true;
		}
		item = item->parent;
	}
	return false;
}
Color RichTextLabel::_find_bgcolor(Item *p_item) {
	Item *item = p_item;
	while (item) {
		if (item->type == ITEM_BGCOLOR) {
			ItemBGColor *color = static_cast<ItemBGColor *>(item);
			return color->color;
		}
		item = item->parent;
	}
	return Color(0, 0, 0, 0);
}
Color RichTextLabel::_find_fgcolor(Item *p_item) {