### 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)

int to_line = main->first_invalid_line.load();
	int lines = main->lines.size();
	return lines - to_line;
}
bool RichTextLabel::is_ready() const {
	const_cast<RichTextLabel *>(this)->_validate_line_caches();
	if (updating.load()) {
		return false;
	}
	return (main->first_invalid_line.load() == (int)main->lines.size() && main->first_resized_line.load() == (int)main->lines.size() && main->first_invalid_font_line.load() == (int)main->lines.size());
}
bool RichTextLabel::is_updating() const {
	return updating.load() || validating.load();
}
void RichTextLabel::set_threaded(bool p_threaded) {
	if (threaded != p_threaded) {
		_stop_thread();
		threaded = p_threaded;
		queue_redraw();
	}
}
bool RichTextLabel::is_threaded() const {
	return threaded;
}
void RichTextLabel::set_progress_bar_delay(int p_delay_ms) {
	progress_delay = p_delay_ms;
}
int RichTextLabel::get_progress_bar_delay() const {