### 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 (!is_inside_tree()) {
		return;
	}
	MutexLock data_lock(data_mutex);
	Rect2 text_rect = _get_text_rect();
	float ctrl_height = get_size().height;
	int fi = main->first_invalid_line.load();
	int total_chars = main->lines[fi].char_offset;
	float old_scroll = vscroll->get_value();
	float total_height = 0;
	if (fi != 0) {
		int sr = MIN(main->first_invalid_font_line.load(), main->first_resized_line.load());
		
		for (int i = main->first_invalid_font_line.load(); i < fi; i++) {
			_update_line_font(main, i, theme_cache.normal_font, theme_cache.normal_font_size);
			main->first_invalid_font_line.store(i);
			if (stop_thread.load()) {
				return;
			}
		}
		
		if (sr != 0) {
			total_height = _calculate_line_vertical_offset(main->lines[sr - 1]);
		}