Rdump crashes on fieldtype `bytes` with the value `None`

[respondersGY]

If a record has a field with the type bytes and value of None, rdump will fail because it cannot base64 decode None.

target-query -f notifications -t <TARGET> -j > notifications.json
rdump notifications.json -c 1
  File "flow.record/flow/record/jsonpacker.py", line 102, in unpack_obj
    obj[field_name] = base64.b64decode(obj[field_name])
                      ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
[...]
    raise TypeError("argument should be a bytes-like object or ASCII "
                    "string, not %r" % s.__class__.__name__) from None
TypeError: argument should be a bytes-like object or ASCII string, not 'NoneType'
XXXX [error    ] rdump encountered a fatal error: argument should be a bytes-like object or ASCII string, not 'NoneType' [flow.record.tools.rdump]

This occurred while using several Dissect Target plugins likenotifications, browser.passwords, etc.

[respondersGY]

@yunzheng thank you for fixing the issue