### 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)

_stop_thread();
	MutexLock data_lock(data_mutex);
	current = main;
	current_frame = main;
}
void RichTextLabel::clear() {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	main->_clear_children();
	current = main;
	current_frame = main;
	main->lines.clear();
	main->lines.resize(1);
	main->first_invalid_line.store(0);
	selection.click_frame = nullptr;
	selection.click_item = nullptr;
	deselect();
	current_idx = 1;
	current_char_ofs = 0;
	if (scroll_follow) {
		scroll_following = true;
	}
	if (fit_content) {
		update_minimum_size();
	}
}
void RichTextLabel::set_tab_size(int p_spaces) {