### 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 (!b->is_double_click() && !scroll_updated && !selection.active) {
					Item *c_item = nullptr;
					bool outside = true;
					_find_click(main, b->get_position(), nullptr, nullptr, &c_item, nullptr, &outside, true);
					if (c_item) {
						Variant meta;
						if (!outside && _find_meta(c_item, &meta)) {
							
							emit_signal(SNAME("meta_clicked"), meta);
						}
					}
				}
			}
		}
		if (b->get_button_index() == MouseButton::WHEEL_UP) {
			if (scroll_active) {
				vscroll->set_value(vscroll->get_value() - vscroll->get_page() * b->get_factor() * 0.5 / 8);
			}
		}
		if (b->get_button_index() == MouseButton::WHEEL_DOWN) {
			if (scroll_active) {
				vscroll->set_value(vscroll->get_value() + vscroll->get_page() * b->get_factor() * 0.5 / 8);
			}
		}
		if (b->get_button_index() == MouseButton::RIGHT && context_menu_enabled) {
			_update_context_menu();
			menu->set_position(get_screen_position() + b->get_position());
			menu->reset_size();