Fix binary operations Index by Series.

[ueshin]

The binary operations between Index and Series raise an error when the operations are on Index and Series in this order and the Series has no name.

>>> ks.Index([1, 2, 3]) + ks.Series([10, 20, 30])
Traceback (most recent call last):
...
TypeError: object of type 'NoneType' has no len()

>>> ks.Index([1, 2, 3]) - ks.Series([10, 20, 30])
Traceback (most recent call last):
...
TypeError: object of type 'NoneType' has no len()

>>> ks.Index([1, 2, 3]) / ks.Series([10, 20, 30])
Traceback (most recent call last):
...
TypeError: object of type 'NoneType' has no len()

>>> ks.Index([1, 2, 3]) * ks.Series([10, 20, 30])
Traceback (most recent call last):
...
TypeError: object of type 'NoneType' has no len()

Resolves #2045.

[codecov-io]

Codecov Report

Merging #2046 (3687b30) into master (dca91a4) will decrease coverage by 2.42%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2046      +/-   ##
==========================================
- Coverage   94.71%   92.29%   -2.43%     
==========================================
  Files          54       54              
  Lines       11497    11388     -109     
==========================================
- Hits        10889    10510     -379     
- Misses        608      878     +270     

Impacted Files	Coverage Δ
databricks/koalas/base.py	97.24% <100.00%> (-0.02%)	⬇️
databricks/koalas/plot/plotly.py	9.47% <0.00%> (-87.37%)	⬇️
...bricks/koalas/tests/plot/test_frame_plot_plotly.py	23.33% <0.00%> (-76.67%)	⬇️
...ricks/koalas/tests/plot/test_series_plot_plotly.py	26.92% <0.00%> (-71.26%)	⬇️
databricks/koalas/__init__.py	85.33% <0.00%> (-6.67%)	⬇️
databricks/koalas/plot/core.py	91.49% <0.00%> (-2.28%)	⬇️
databricks/conftest.py	98.30% <0.00%> (-1.70%)	⬇️
databricks/koalas/testing/utils.py	80.00% <0.00%> (-1.32%)	⬇️
databricks/koalas/generic.py	92.07% <0.00%> (-1.18%)	⬇️
databricks/koalas/indexes/multi.py	95.37% <0.00%> (-0.44%)	⬇️
... and 18 more

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[itholic]

Nice fix! LGTM.

[itholic]

FYI: after merging this, I'm going to create a PR for supporting binary operations with list-like Python objects we discussed in #2022 (comment) !

[ueshin]

Thanks! merging.