### 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)

current_line = p_search_previous ? ending_line - 1 : ending_line + 1;
	} else if (p_search_previous) {
		current_line = ending_line;
		ending_line = 0;
	}
	
	while (current_line != ending_line) {
		
		if (current_line < 0) {
			current_line = to_line - 1;
		} else if (current_line >= to_line) {
			current_line = 0;
		}
		if (_search_line(main, current_line, p_string, char_idx, p_search_previous)) {
			scroll_to_selection();
			queue_redraw();
			return true;
		}
		if (current_line != ending_line) {
			p_search_previous ? current_line-- : current_line++;
		}
	}
	if (p_from_selection && selection.active) {
		
		return _search_line(main, current_line, p_string, char_idx, p_search_previous);
	} else {
		return false;
	}
}
String RichTextLabel::_get_line_text(ItemFrame *p_frame, int p_line, Selection p_selection) const {