### 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)

} break;
					default:
						break;
				}
			}
		}
		const Glyph *glyphs = TS->shaped_text_get_glyphs(rid);
		int gl_size = TS->shaped_text_get_glyph_count(rid);
		Vector2 gloff = off;
		
		int processed_glyphs_ol = r_processed_glyphs;
		for (int i = 0; i < gl_size; i++) {
			Item *it = _get_item_at_pos(it_from, it_to, glyphs[i].start);
			int size = _find_outline_size(it, p_outline_size);
			Color font_color = _find_color(it, p_base_color);
			Color font_outline_color = _find_outline_color(it, p_outline_color);
			Color font_shadow_color = p_font_shadow_color;
			if ((size <= 0 || font_outline_color.a == 0) && (font_shadow_color.a == 0)) {
				gloff.x += glyphs[i].advance;
				continue;
			}
			
			ItemFade *fade = nullptr;
			Item *fade_item = it;
			while (fade_item) {
				if (fade_item->type == ITEM_FADE) {
					fade = static_cast<ItemFade *>(fade_item);
					break;
				}
				fade_item = fade_item->parent;