### 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)

_validate_line_caches();
	int to_line = main->first_invalid_line.load();
	if (0 <= p_paragraph && p_paragraph < to_line) {
		return main->lines[p_paragraph].offset.y;
	}
	return 0;
}
int RichTextLabel::get_line_count() const {
	const_cast<RichTextLabel *>(this)->_validate_line_caches();
	int line_count = 0;
	int to_line = main->first_invalid_line.load();
	for (int i = 0; i < to_line; i++) {
		MutexLock lock(main->lines[i].text_buf->get_mutex());
		line_count += main->lines[i].text_buf->get_line_count();
	}
	return line_count;
}
int RichTextLabel::get_visible_line_count() const {