### 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);
	ItemStrikethrough *item = memnew(ItemStrikethrough);
	_add_item(item, true);
}
void RichTextLabel::push_paragraph(HorizontalAlignment p_alignment, Control::TextDirection p_direction, const String &p_language, TextServer::StructuredTextParser p_st_parser, BitField<TextServer::JustificationFlag> p_jst_flags, const PackedFloat32Array &p_tab_stops) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemParagraph *item = memnew(ItemParagraph);
	item->alignment = p_alignment;
	item->direction = p_direction;
	item->language = p_language;
	item->st_parser = p_st_parser;
	item->jst_flags = p_jst_flags;
	item->tab_stops = p_tab_stops;
	_add_item(item, true, true);
}
void RichTextLabel::push_indent(int p_level) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ERR_FAIL_COND(p_level < 0);
	ItemIndent *item = memnew(ItemIndent);
	item->level = p_level;
	_add_item(item, true, true);
}
void RichTextLabel::push_list(int p_level, ListType p_list, bool p_capitalize, const String &p_bullet) {