Simplify passing of framerate

PipeWireBaseEncodedStream::maxFramerate has a getter for the framerate
which returns the value of a std::optional without checking which is
very prone to error.

Rather than adding more optionals we can make Fractional a nullable
type, a denominator of 0 is not a valid value. If Fraction can specify a
null type we don't need optional anywhere.

A comparison operator is also added.