### 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)

set_current_thread_safe_for_nodes(true);
	_process_line_caches();
	updating.store(false);
	call_deferred(SNAME("thread_end"));
}
void RichTextLabel::_thread_end() {
	set_physics_process_internal(false);
	if (is_visible_in_tree()) {
		queue_redraw();
	}
}
void RichTextLabel::_stop_thread() {
	if (threaded) {
		stop_thread.store(true);
		if (task != WorkerThreadPool::INVALID_TASK_ID) {
			WorkerThreadPool::get_singleton()->wait_for_task_completion(task);
			task = WorkerThreadPool::INVALID_TASK_ID;
		}
	}
}
int RichTextLabel::get_pending_paragraphs() const {