fix: nbytes property of ak.Record

[pfackeldey]

This was uncovered by #3348 . (This bug is 3 years old, I suppose .nbytes was never accessed in that time on a ak.Record 😅)

Before:

>>> ak.Record({"foo": [1,2,3]}).nbytes
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../awkward/highlevel.py", line 2200, in __getattr__
    return super().__getattribute__(where)
  File ".../awkward/highlevel.py", line 2012, in nbytes
    return self._layout.nbytes
AttributeError: 'Record' object has no attribute 'nbytes'

Now:

>>> ak.Record({"foo": [1,2,3]}).nbytes
>>> 24

[jpivarski]

Good catch!