### 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();
	if (p_paragraph <= 0) {
		vscroll->set_value(0);
	} else if (p_paragraph >= main->first_invalid_line.load()) {
		vscroll->set_value(vscroll->get_max());
	} else {
		vscroll->set_value(main->lines[p_paragraph].offset.y);
	}
}
int RichTextLabel::get_paragraph_count() const {
	return current_frame->lines.size();
}
int RichTextLabel::get_visible_paragraph_count() const {