Fix default value ro_freq is null causing resonator scan to fail

[Odrenal]

Steps to reproduce

1) use an new qubit object 2) run find_resonator_frequency() 3) watch it crash

Expected behaviour

function does resonator scan

Actual behaviour

function crashes with TypeError: unsupported operand type(s) for -: 'Nonetype' and 'float'.

[AdriaanRol]

👋 Who are you @Odrenal ?

[MiguelSMoreira]

This implementation of the resonator scan function already follows the expected behavior for the qubit object