### 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_PARAGRAPH) {
			ItemParagraph *p = static_cast<ItemParagraph *>(item);
			return p->st_parser;
		}
		item = item->parent;
	}
	return st_parser;
}
String RichTextLabel::_find_language(Item *p_item) {
	Item *item = p_item;
	while (item) {
		if (item->type == ITEM_PARAGRAPH) {
			ItemParagraph *p = static_cast<ItemParagraph *>(item);
			return p->language;
		}
		item = item->parent;
	}
	return language;
}
Color RichTextLabel::_find_color(Item *p_item, const Color &p_default_color) {