[BUG]Getting dtypes on an empty cuDF dataframe raises a futurewarning

[VibhuJawa]

Describe the bug

Getting dtypes on an empty dataframe raises a future warning that was confusing to some users. Might be good to look into triaging it and ensuring if we want this behavior or not.

Steps/Code to reproduce bug

>>> import cudf
>>> cudf.DataFrame().dtypes

/datasets/vjawa/miniconda3/envs/cugraph_dev/lib/python3.9/site-packages/cudf/core/dataframe.py:1059: FutureWarning: The default dtype for empty Series will be 'object' instead of 'float64' in a future version. Specify a dtype explicitly to silence this warning.
  return pd.Series(self._dtypes)
Series([], dtype: float64)

[VibhuJawa]

related cugraph issue: rapidsai/cugraph#2619