Add empty DataFrame check to boundary_slice

[jrbourbeau]

Closes #4211. It looks like the bug in #4211 is due to empty DataFrames being passed to boundary_slice() in dask/dataframe/methods.py. This PR adds a check in boundary_slice to handle when empty DataFrames are input. Specifically, the empty DataFrame is returned by boundary_slice. E.g.

>>> df_empty = pd.DataFrame()
>>> boundary_slice(df_empty, 1, 3)
Empty DataFrame
Columns: []
Index: []

I'd be curious to see what other think about this. Definitely open to suggestions.

• Tests added / passed
• Passes flake8 dask

[mrocklin]

This seems great to me. Thanks for tracking this down @jrbourbeau .

Merging in an hour if there are no further comments.

[mrocklin]

Merged. Thanks @jrbourbeau !