### 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)

ERR_FAIL_COND(p_frame == nullptr);
	ERR_FAIL_COND(p_line < 0 || p_line >= (int)p_frame->lines.size());
	Line &l = p_frame->lines[p_line];
	MutexLock lock(l.text_buf->get_mutex());
	RID t = l.text_buf->get_rid();
	int spans = TS->shaped_get_span_count(t);
	for (int i = 0; i < spans; i++) {
		ItemText *it = reinterpret_cast<ItemText *>((uint64_t)TS->shaped_get_span_meta(t, i));
		if (it) {
			Ref<Font> font = p_base_font;
			int font_size = p_base_font_size;
			ItemFont *font_it = _find_font(it);
			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(it);
			if (font_size_it && font_size_it->font_size > 0) {
				font_size = font_size_it->font_size;
			}
			TS->shaped_set_span_update_font(t, i, font->get_rids(), font_size, font->get_opentype_features());
			for (int j = 0; j < TextServer::SPACING_MAX; j++) {
				TS->shaped_text_set_spacing(t, TextServer::SpacingType(j), font->get_spacing(TextServer::SpacingType(j)));
			}