### 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);
	if (current->type == ITEM_TABLE) {
		return;
	}
	ItemNewline *item = memnew(ItemNewline);
	item->line = current_frame->lines.size();
	_add_item(item, false);
	current_frame->lines.resize(current_frame->lines.size() + 1);
	_invalidate_current_line(current_frame);
	queue_redraw();
}
bool RichTextLabel::remove_paragraph(const int p_paragraph) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	if (p_paragraph >= (int)current_frame->lines.size() || p_paragraph < 0) {
		return false;