### 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)

if (underline_meta == p_underline) {
		return;
	}
	underline_meta = p_underline;
	queue_redraw();
}
bool RichTextLabel::is_meta_underlined() const {
	return underline_meta;
}
void RichTextLabel::set_hint_underline(bool p_underline) {
	underline_hint = p_underline;
	queue_redraw();
}
bool RichTextLabel::is_hint_underlined() const {
	return underline_hint;
}
void RichTextLabel::set_offset(int p_pixel) {
	vscroll->set_value(p_pixel);
}
void RichTextLabel::set_scroll_active(bool p_active) {
	if (scroll_active == p_active) {
		return;
	}
	scroll_active = p_active;
	vscroll->set_drag_node_enabled(p_active);
	queue_redraw();
}
bool RichTextLabel::is_scroll_active() const {