### 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 (_find_underline(it) || (_find_meta(it, nullptr) && underline_meta)) {
				if (!ul_started) {
					ul_started = true;
					ul_start = p_ofs + Vector2(off.x, off.y);
					ul_color = font_color;
					ul_color.a *= 0.5;
				}
			} else if (ul_started) {
				ul_started = false;
				float y_off = TS->shaped_text_get_underline_position(rid);
				float underline_width = MAX(1.0, TS->shaped_text_get_underline_thickness(rid) * theme_cache.base_scale);
				draw_line(ul_start + Vector2(0, y_off), p_ofs + Vector2(off.x, off.y + y_off), ul_color, underline_width);
			}
			if (_find_hint(it, nullptr) && underline_hint) {
				if (!dot_ul_started) {
					dot_ul_started = true;
					dot_ul_start = p_ofs + Vector2(off.x, off.y);
					dot_ul_color = font_color;
					dot_ul_color.a *= 0.5;
				}
			} else if (dot_ul_started) {
				dot_ul_started = false;
				float y_off = TS->shaped_text_get_underline_position(rid);
				float underline_width = MAX(1.0, TS->shaped_text_get_underline_thickness(rid) * theme_cache.base_scale);
				draw_dashed_line(dot_ul_start + Vector2(0, y_off), p_ofs + Vector2(off.x, off.y + y_off), dot_ul_color, underline_width, MAX(2.0, underline_width * 2));
			}
			if (_find_strikethrough(it)) {
				if (!st_started) {
					st_started = true;
					st_start = p_ofs + Vector2(off.x, off.y);