### 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)

ERR_FAIL_COND_V(current->type != ITEM_TABLE, -1);
	ItemTable *table = static_cast<ItemTable *>(current);
	return table->subitems.size() % table->columns.size();
}
void RichTextLabel::pop() {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_NULL(current->parent);
	if (current->type == ITEM_FRAME) {
		current_frame = static_cast<ItemFrame *>(current)->parent_frame;
	}
	current = current->parent;
}
void RichTextLabel::pop_context() {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_NULL(current->parent);
	while (current->parent && current != main) {
		if (current->type == ITEM_FRAME) {
			current_frame = static_cast<ItemFrame *>(current)->parent_frame;
		} else if (current->type == ITEM_CONTEXT) {
			current = current->parent;
			return;
		}
		current = current->parent;
	}
}
void RichTextLabel::pop_all() {