Elixir 1.18 compatibility

[zrzka]

This PR fixes warnings being produced by Elixir 1.18.4 forbidding us to use --warnings-as-errors.

Simple usage like File.write!(printed_pdf, Fast64.decode64(pdf_data)) produces the following error ...

Compiling 28 files (.ex)
    warning: incompatible types given to File.write!/2:

        File.write!(printed_pdf, Fast64.decode64(pdf_data))

    given types:

        dynamic(), none()

    the 2nd argument is empty (often represented as none()), most likely because it is the result of an expression that always fails, such as a `raise` or a previous invalid call. This causes any function called with this value to fail

... which forbids us from keeping --warnings-as-errors.