### 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)

return context_menu_enabled;
}
void RichTextLabel::set_shortcut_keys_enabled(bool p_enabled) {
	shortcut_keys_enabled = p_enabled;
}
bool RichTextLabel::is_shortcut_keys_enabled() const {
	return shortcut_keys_enabled;
}
PopupMenu *RichTextLabel::get_menu() const {
	if (!menu) {
		const_cast<RichTextLabel *>(this)->_generate_context_menu();
	}
	return menu;
}
bool RichTextLabel::is_menu_visible() const {
	return menu && menu->is_visible();
}
String RichTextLabel::get_selected_text() const {
	if (!selection.active || !selection.enabled) {
		return "";
	}
	String txt;
	int to_line = main->first_invalid_line.load();
	for (int i = 0; i < to_line; i++) {
		txt += _get_line_text(main, i, selection);
	}
	return txt;
}
void RichTextLabel::deselect() {