### 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)

int64_t n = p_num;
	int chars = 0;
	do {
		n /= 24;
		chars++;
	} while (n);
	String s;
	s.resize(chars + 1);
	char32_t *c = s.ptrw();
	c[chars] = 0;
	n = p_num;
	do {
		int mod = ABS(n % 24);
		char a = (p_capitalize ? 'A' : 'a');
		c[--chars] = a + mod - 1;
		n /= 24;
	} while (n);
	return s;
}
void RichTextLabel::_update_line_font(ItemFrame *p_frame, int p_line, const Ref<Font> &p_base_font, int p_base_font_size) {