### 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)

line = p_text.substr(pos, end - pos);
		}
		if (line.length() > 0) {
			if (current->subitems.size() && current->subitems.back()->get()->type == ITEM_TEXT) {
				
				ItemText *ti = static_cast<ItemText *>(current->subitems.back()->get());
				ti->text += line;
				_invalidate_current_line(main);
			} else {
				
				ItemText *item = memnew(ItemText);
				item->text = line;
				_add_item(item, false);
			}
		}
		if (eol) {
			ItemNewline *item = memnew(ItemNewline);
			item->line = current_frame->lines.size();
			_add_item(item, false);
			current_frame->lines.resize(current_frame->lines.size() + 1);
			if (item->type != ITEM_NEWLINE) {
				current_frame->lines[current_frame->lines.size() - 1].from = item;
			}
			_invalidate_current_line(current_frame);
		}
		pos = end + 1;
	}
	queue_redraw();
}
void RichTextLabel::_add_item(Item *p_item, bool p_enter, bool p_ensure_newline) {