### 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)

ERR_FAIL_COND(theme_cache.bold_italics_font.is_null());
	_push_def_font(BOLD_ITALICS_FONT);
}
void RichTextLabel::push_italics() {
	ERR_FAIL_COND(theme_cache.italics_font.is_null());
	ItemFont *item_font = _find_font(current);
	_push_def_font((item_font && item_font->def_font == BOLD_FONT) ? BOLD_ITALICS_FONT : ITALICS_FONT);
}
void RichTextLabel::push_mono() {
	ERR_FAIL_COND(theme_cache.mono_font.is_null());
	_push_def_font(MONO_FONT);
}
void RichTextLabel::push_font_size(int p_font_size) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemFontSize *item = memnew(ItemFontSize);
	item->font_size = p_font_size;
	_add_item(item, true);
}
void RichTextLabel::push_outline_size(int p_ol_size) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemOutlineSize *item = memnew(ItemOutlineSize);
	item->outline_size = p_ol_size;
	_add_item(item, true);
}
void RichTextLabel::push_color(const Color &p_color) {