### 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)

Item *c_item = nullptr;
	bool outside;
	const_cast<RichTextLabel *>(this)->_find_click(main, p_pos, nullptr, nullptr, &c_item, nullptr, &outside, true);
	String description;
	if (c_item && !outside && const_cast<RichTextLabel *>(this)->_find_hint(c_item, &description)) {
		return description;
	} else {
		return Control::get_tooltip(p_pos);
	}
}
void RichTextLabel::_find_frame(Item *p_item, ItemFrame **r_frame, int *r_line) {