fix(phpstan): update function parameter data types

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 (closed)