### 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)

if (selection.click_item) {
		return CURSOR_IBEAM;
	}
	Item *item = nullptr;
	bool outside = true;
	const_cast<RichTextLabel *>(this)->_find_click(main, p_pos, nullptr, nullptr, &item, nullptr, &outside, true);
	if (item && !outside && const_cast<RichTextLabel *>(this)->_find_meta(item, nullptr)) {
		return CURSOR_POINTING_HAND;
	}
	return get_default_cursor_shape();
}
void RichTextLabel::gui_input(const Ref<InputEvent> &p_event) {