### 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)

p_item->parent = current;
	p_item->E = current->subitems.push_back(p_item);
	p_item->index = current_idx++;
	p_item->char_ofs = current_char_ofs;
	if (p_item->type == ITEM_TEXT) {
		ItemText *t = static_cast<ItemText *>(p_item);
		current_char_ofs += t->text.length();
	} else if (p_item->type == ITEM_IMAGE) {
		current_char_ofs++;
	}
	if (p_enter) {
		current = p_item;
	}
	if (p_ensure_newline) {
		Item *from = current_frame->lines[current_frame->lines.size() - 1].from;
		
		if (_find_layout_subitem(from, p_item)) {
			_invalidate_current_line(current_frame);
			current_frame->lines.resize(current_frame->lines.size() + 1);
		}
	}
	if (current_frame->lines[current_frame->lines.size() - 1].from == nullptr) {
		current_frame->lines[current_frame->lines.size() - 1].from = p_item;
	}
	p_item->line = current_frame->lines.size() - 1;