### 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_TABLE);
	ItemTable *table = static_cast<ItemTable *>(current);
	ERR_FAIL_INDEX(p_column, (int)table->columns.size());
	table->columns[p_column].expand = p_expand;
	table->columns[p_column].expand_ratio = p_ratio;
}
void RichTextLabel::set_cell_row_background_color(const Color &p_odd_row_bg, const Color &p_even_row_bg) {
	_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->odd_row_bg = p_odd_row_bg;
	cell->even_row_bg = p_even_row_bg;
}
void RichTextLabel::set_cell_border_color(const Color &p_color) {
	_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->border = p_color;
}
void RichTextLabel::set_cell_size_override(const Size2 &p_min_size, const Size2 &p_max_size) {