### 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)

if (p_region.has_area()) {
				item->size.height = p_region.get_size().height * p_width / p_region.get_size().width;
			} else {
				item->size.height = p_image->get_height() * p_width / p_image->get_width();
			}
		}
	} else {
		if (p_height > 0) {
			
			item->size.height = p_height;
			
			if (p_region.has_area()) {
				item->size.width = p_region.get_size().width * p_height / p_region.get_size().height;
			} else {
				item->size.width = p_image->get_width() * p_height / p_image->get_height();
			}
		} else {
			if (p_region.has_area()) {
				
				item->size = p_region.get_size();
			} else {
				
				item->size = p_image->get_size();
			}
		}
	}
	_add_item(item, false);
}
void RichTextLabel::add_newline() {