### 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_level < 0);
	ItemList *item = memnew(ItemList);
	item->list_type = p_list;
	item->level = p_level;
	item->capitalize = p_capitalize;
	item->bullet = p_bullet;
	_add_item(item, true, true);
}
void RichTextLabel::push_meta(const Variant &p_meta) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemMeta *item = memnew(ItemMeta);
	item->meta = p_meta;
	_add_item(item, true);
}
void RichTextLabel::push_hint(const String &p_string) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemHint *item = memnew(ItemHint);
	item->description = p_string;
	_add_item(item, true);
}
void RichTextLabel::push_table(int p_columns, InlineAlignment p_alignment, int p_align_to_row) {