### 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)

menu->popup();
			grab_focus();
		}
	}
	Ref<InputEventPanGesture> pan_gesture = p_event;
	if (pan_gesture.is_valid()) {
		if (scroll_active) {
			vscroll->set_value(vscroll->get_value() + vscroll->get_page() * pan_gesture->get_delta().y * 0.5 / 8);
		}
		return;
	}
	Ref<InputEventKey> k = p_event;
	if (k.is_valid()) {
		if (k->is_pressed()) {
			bool handled = false;
			if (k->is_action("ui_page_up", true) && vscroll->is_visible_in_tree()) {
				vscroll->set_value(vscroll->get_value() - vscroll->get_page());
				handled = true;
			}
			if (k->is_action("ui_page_down", true) && vscroll->is_visible_in_tree()) {
				vscroll->set_value(vscroll->get_value() + vscroll->get_page());
				handled = true;
			}
			if (k->is_action("ui_up", true) && vscroll->is_visible_in_tree()) {
				vscroll->set_value(vscroll->get_value() - theme_cache.normal_font->get_height(theme_cache.normal_font_size));
				handled = true;
			}
			if (k->is_action("ui_down", true) && vscroll->is_visible_in_tree()) {
				vscroll->set_value(vscroll->get_value() + theme_cache.normal_font->get_height(theme_cache.normal_font_size));
				handled = true;