### 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)

}
		const Size2 line_size = l.text_buf->get_line_size(line);
		if (p_ofs.y + off.y + line_size.y <= 0) {
			off.y += line_size.y;
			continue;
		}
		float width = l.text_buf->get_width();
		float length = line_size.x;
		
		line_count++;
		if (rtl) {
			off.x = p_width - l.offset.x - width;
			if (!lrtl && p_frame == main) { 
				off.x -= scroll_w;
			}
		} else {
			off.x = l.offset.x;
			if (lrtl && p_frame == main) { 
				off.x += scroll_w;
			}
		}
		
		switch (l.text_buf->get_alignment()) {
			case HORIZONTAL_ALIGNMENT_FILL:
			case HORIZONTAL_ALIGNMENT_LEFT: {
				if (rtl) {
					off.x += width - length;
				}
			} break;
			case HORIZONTAL_ALIGNMENT_CENTER: {