### 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)

int processed_glyphs = 0;
			while (ofs.y < size.height && from_line < to_line) {
				MutexLock lock(main->lines[from_line].text_buf->get_mutex());
				visible_paragraph_count++;
				visible_line_count += _draw_line(main, from_line, ofs, text_rect.size.x, theme_cache.default_color, theme_cache.outline_size, theme_cache.font_outline_color, theme_cache.font_shadow_color, theme_cache.shadow_outline_size, shadow_ofs, processed_glyphs);
				ofs.y += main->lines[from_line].text_buf->get_size().y + main->lines[from_line].text_buf->get_line_count() * theme_cache.line_separation;
				from_line++;
			}
		} break;
		case NOTIFICATION_INTERNAL_PROCESS: {
			if (is_visible_in_tree()) {
				if (!is_ready()) {
					return;
				}
				double dt = get_process_delta_time();
				_update_fx(main, dt);
				queue_redraw();
			}
		} break;
		case NOTIFICATION_FOCUS_EXIT: {
			if (deselect_on_focus_loss_enabled) {
				deselect();
			}
		} break;
		case NOTIFICATION_DRAG_END: {
			selection.drag_attempt = false;
		} break;
	}
}
Control::CursorShape RichTextLabel::get_cursor_shape(const Point2 &p_pos) const {