### 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)

_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type != ITEM_FRAME);
	ItemFrame *cell = static_cast<ItemFrame *>(current);
	ERR_FAIL_COND(!cell->cell);
	cell->min_size_over = p_min_size;
	cell->max_size_over = p_max_size;
}
void RichTextLabel::set_cell_padding(const Rect2 &p_padding) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type != ITEM_FRAME);
	ItemFrame *cell = static_cast<ItemFrame *>(current);
	ERR_FAIL_COND(!cell->cell);
	cell->padding = p_padding;
}
void RichTextLabel::push_cell() {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type != ITEM_TABLE);
	ItemFrame *item = memnew(ItemFrame);
	item->parent_frame = current_frame;
	_add_item(item, true);
	current_frame = item;
	item->cell = true;
	item->lines.resize(1);
	item->lines[0].from = nullptr;
	item->first_invalid_line.store(0); 
}
int RichTextLabel::get_current_table_column() const {