set eeprom default values to `None` or nothing

[farzadpanahi]

I noticed that default eeprom values are set like the following:

, serial='This is Serial Number', model='This is Model Number', cm_part_number='This is cm board part number', tb_part_number='This is tb part number', cm4_part_number='This is cm4 part number')

let's change that and set them to None, otherwise this will be problematic in future because we will have some bad values set by default for lots of edgepis :D

[sjpark608]

For data_key and config_key. They are dataclass which has attributes private_key and certificate.

Should I change change the whole dataclass to None or just each attributes to None. I think changing each attibute to None makes more sense. This way, the user doesn't have to import dataclass to populate.