I believe this empty check is incorrect, because it will trigger an AssertionError on the string '0', which could be a perfectly fine result (e.g. in Polish decimal localisation). I'd suggest removing these assertions altogether; they occur in multiple formatters!
https://github.com/moneyphp/money/blob/df7234fcb412d337b309e1b20d00f977614f82d5/src/Formatter/IntlMoneyFormatter.php#L61
I believe this empty check is incorrect, because it will trigger an AssertionError on the string '0', which could be a perfectly fine result (e.g. in Polish decimal localisation). I'd suggest removing these assertions altogether; they occur in multiple formatters!