### 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_UNDERLINE) {
			return true;
		}
		item = item->parent;
	}
	return false;
}
bool RichTextLabel::_find_strikethrough(Item *p_item) {
	Item *item = p_item;
	while (item) {
		if (item->type == ITEM_STRIKETHROUGH) {
			return true;
		}
		item = item->parent;
	}
	return false;
}
void RichTextLabel::_fetch_item_fx_stack(Item *p_item, Vector<ItemFX *> &r_stack) {
	Item *item = p_item;
	while (item) {
		if (item->type == ITEM_CUSTOMFX || item->type == ITEM_SHAKE || item->type == ITEM_WAVE || item->type == ITEM_TORNADO || item->type == ITEM_RAINBOW || item->type == ITEM_PULSE) {
			r_stack.push_back(static_cast<ItemFX *>(item));
		}
		item = item->parent;
	}
}
void RichTextLabel::_normalize_subtags(Vector<String> &subtags) {