### 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)

const List<Ref<InputEvent>> *events = InputMap::get_singleton()->action_get_events(p_action);
	if (!events) {
		return Key::NONE;
	}
	// 
	const List<Ref<InputEvent>>::Element *first_event = events->front();
	if (!first_event) {
		return Key::NONE;
	}
	const Ref<InputEventKey> event = first_event->get();
	if (event.is_null()) {
		return Key::NONE;
	}
	// 
	if (event->get_physical_keycode() != Key::NONE) {
		return event->get_physical_keycode_with_modifiers();
	} else {
		return event->get_keycode_with_modifiers();
	}
}
void RichTextLabel::menu_option(int p_option) {