### 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);
	ERR_FAIL_COND(p_columns < 1);
	ItemTable *item = memnew(ItemTable);
	item->columns.resize(p_columns);
	item->total_width = 0;
	item->inline_align = p_alignment;
	item->align_to_row = p_align_to_row;
	for (int i = 0; i < (int)item->columns.size(); i++) {
		item->columns[i].expand = false;
		item->columns[i].expand_ratio = 1;
	}
	_add_item(item, true, false);
}
void RichTextLabel::push_fade(int p_start_index, int p_length) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemFade *item = memnew(ItemFade);
	item->starting_index = p_start_index;
	item->length = p_length;
	_add_item(item, true);
}
void RichTextLabel::push_shake(int p_strength = 10, float p_rate = 24.0f, bool p_connected = true) {