### 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)

}
		it = _get_next_item(it, true);
	}
	if (!from_item || !to_item) {
		return;
	}
	ItemFrame *from_frame = nullptr;
	int from_line = 0;
	_find_frame(from_item, &from_frame, &from_line);
	if (!from_frame) {
		return;
	}
	ItemFrame *to_frame = nullptr;
	int to_line = 0;
	_find_frame(to_item, &to_frame, &to_line);
	if (!to_frame) {
		return;
	}
	selection.from_line = from_line;
	selection.from_frame = from_frame;
	selection.from_char = 0;
	selection.from_item = from_item;
	selection.to_line = to_line;
	selection.to_frame = to_frame;
	selection.to_char = to_frame->lines[to_line].char_count;
	selection.to_item = to_item;
	selection.active = true;
	queue_redraw();
}
bool RichTextLabel::is_selection_enabled() const {