Do not call get on completed_session unless it's a dict

[steinmn]

Does not fix the underlying problem of signedsession being null in #343, but at least it doesn't crash the sensor setup.

[steinmn]

Should maybe start type_ocmf with an if not data-block. Do we want it to return None or {}?

[steinmn]

What loglevel do we want for this @sveinse? Since we have not seen this before, maybe it's enough to set it to warning?

[steinmn]

https://github.com/custom-components/zaptec/actions/runs/18541493246/job/52849850821 reproduces the error message by including "SignedSession":null in the input to the complete_session-type-conversion. Still not sure why it is null, but in the description of the completed_session-object in https://docs.zaptec.com/reference/api_chargehistory_get, SignedSession is listed as string | null, so I guess this is expected behaviour?