### 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);
	ItemFont *item = memnew(ItemFont);
	item->def_font = p_def_font;
	item->def_size = true;
	_add_item(item, true);
}
void RichTextLabel::push_font(const Ref<Font> &p_font, int p_size) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ERR_FAIL_COND(p_font.is_null());
	ItemFont *item = memnew(ItemFont);
	item->font = p_font;
	item->font_size = p_size;
	_add_item(item, true);
}
void RichTextLabel::push_normal() {
	ERR_FAIL_COND(theme_cache.normal_font.is_null());
	_push_def_font(NORMAL_FONT);
}
void RichTextLabel::push_bold() {
	ERR_FAIL_COND(theme_cache.bold_font.is_null());
	ItemFont *item_font = _find_font(current);
	_push_def_font((item_font && item_font->def_font == ITALICS_FONT) ? BOLD_ITALICS_FONT : BOLD_FONT);
}
void RichTextLabel::push_bold_italics() {