[FR] Check if input to model can be None by default

[stroblme]

Is your feature request related to a problem? Please describe.
The model implementation can handle None inputs which would allow this argument being optional.
However, this could interfere with e.g. the coefficient calculation as pennylane searches for non-kwargs when looking for inputs or parameters.

Describe the solution you'd like
It would be nice to have input being None as default as in multiple cases, this would simplify calls to the model.

Describe alternatives you've considered
Keep as is.

Additional context
N/A

[stroblme]

solved with #58