CLN: x-array test warnings
Remove these warnings:

```
pandas/tests/generic/test_to_xarray.py:102
  /home/travis/build/pandas-dev/pandas/pandas/tests/generic/test_to_xarray.py:102: DeprecationWarning: The default dtype for empty Series will be 'object' instead of 'float64' in a future version. Specify a dtype explicitly to silence this warning.
    s = Series(range(len(indices)), index=indices)
```

https://travis-ci.org/github/pandas-dev/pandas/jobs/675056013