### 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)

case ITEM_TABLE: {
				offset += 1;
			} break;
			default:
				break;
		}
	}
	return p_item_from;
}
String RichTextLabel::_roman(int p_num, bool p_capitalize) const {
	if (p_num > 3999) {
		return "ERR";
	};
	String s;
	if (p_capitalize) {
		const String roman_M[] = { "", "M", "MM", "MMM" };
		const String roman_C[] = { "", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM" };
		const String roman_X[] = { "", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC" };
		const String roman_I[] = { "", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX" };
		s = roman_M[p_num / 1000] + roman_C[(p_num % 1000) / 100] + roman_X[(p_num % 100) / 10] + roman_I[p_num % 10];
	} else {
		const String roman_M[] = { "", "m", "mm", "mmm" };
		const String roman_C[] = { "", "c", "cc", "ccc", "cd", "d", "dc", "dcc", "dccc", "cm" };
		const String roman_X[] = { "", "x", "xx", "xxx", "xl", "l", "lx", "lxx", "lxxx", "xc" };
		const String roman_I[] = { "", "i", "ii", "iii", "iv", "v", "vi", "vii", "viii", "ix" };
		s = roman_M[p_num / 1000] + roman_C[(p_num % 1000) / 100] + roman_X[(p_num % 100) / 10] + roman_I[p_num % 10];
	}
	return s;
}
String RichTextLabel::_letters(int p_num, bool p_capitalize) const {