### 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 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());
		if ((line_count <= p_line) && (p_line <= line_count + main->lines[i].text_buf->get_line_count())) {
			float line_offset = 0.f;
			for (int j = 0; j < p_line - line_count; j++) {
				line_offset += main->lines[i].text_buf->get_line_size(j).y + theme_cache.line_separation;
			}
			return main->lines[i].offset.y + line_offset;
		}
		line_count += main->lines[i].text_buf->get_line_count();
	}
	return 0;
}
float RichTextLabel::get_paragraph_offset(int p_paragraph) {