[BUG] Series.rename(None) does not rename existing name to None

[mroeschke]

Describe the bug
Series.rename(None) does not rename existing name to None

Steps/Code to reproduce bug

In [1]: from cudf import *

In [2]: ser = Series([1, 2], name="foo")

In [3]: ser.rename(None)
Out[3]: 
0    1
1    2
Name: foo, dtype: int64

In [8]: ser.to_pandas().rename(None)
Out[8]: 
0    1
1    2
dtype: int64

Expected behavior

In [3]: ser.rename(None)
Out[3]: 
0    1
1    2
dtype: int64

The default index=None probably needs to be change to a no default sentinel.

Environment overview (please complete the following information)

• Environment location: Bare-metal
• Method of cuDF install: conda
  • If method of install is [Docker], provide docker pull & docker run commands used

Environment details
Please run and paste the output of the cudf/print_env.sh script here, to gather any other relevant environment details

Additional context
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