### 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)

theme_cache.table_h_separation = get_theme_constant(SNAME("table_h_separation"));
	theme_cache.table_v_separation = get_theme_constant(SNAME("table_v_separation"));
	theme_cache.table_odd_row_bg = get_theme_color(SNAME("table_odd_row_bg"));
	theme_cache.table_even_row_bg = get_theme_color(SNAME("table_even_row_bg"));
	theme_cache.table_border = get_theme_color(SNAME("table_border"));
	theme_cache.base_scale = get_theme_default_base_scale();
}
void RichTextLabel::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_MOUSE_EXIT: {
			if (meta_hovering) {
				meta_hovering = nullptr;
				emit_signal(SNAME("meta_hover_ended"), current_meta);
				current_meta = false;
				queue_redraw();
			}
		} break;
		case NOTIFICATION_RESIZED: {
			_stop_thread();
			main->first_resized_line.store(0); 
			queue_redraw();
		} break;
		case NOTIFICATION_THEME_CHANGED: {
			_stop_thread();
			main->first_invalid_font_line.store(0); 
			queue_redraw();
		} break;
		case NOTIFICATION_ENTER_TREE: {
			_stop_thread();
			if (!text.is_empty()) {