### 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)

if (is_localizing_numeral_system()) {
				segment = TS->format_number(segment, _find_language(l.from));
			}
		} else if (list_items[i]->list_type == LIST_LETTERS) {
			segment = _letters(list_index[i], list_items[i]->capitalize);
		} else if (list_items[i]->list_type == LIST_ROMAN) {
			segment = _roman(list_index[i], list_items[i]->capitalize);
		}
		if (rtl) {
			prefix = prefix + segment;
		} else {
			prefix = segment + prefix;
		}
	}
	if (!prefix.is_empty()) {
		Ref<Font> font = theme_cache.normal_font;
		int font_size = theme_cache.normal_font_size;
		ItemFont *font_it = _find_font(l.from);
		if (font_it) {
			if (font_it->font.is_valid()) {
				font = font_it->font;
			}
			if (font_it->font_size > 0) {
				font_size = font_it->font_size;
			}
		}
		ItemFontSize *font_size_it = _find_font_size(l.from);
		if (font_size_it && font_size_it->font_size > 0) {
			font_size = font_size_it->font_size;
		}