This commit changes the data type of the function parameters across various files. The
modifications are made in the 'phpstan-baseline.neon', 'AbstractCurrency.php',
'CurrencyInterface.php', and 'Handler.php' files. Certain function parameters which could have been
null are now specifically marked as available for null input. Improving the type hinting of these
functions enhances code readability, allows for better static analysis and reduces the scope for
bugs in the future.

Related: #9