Explicitly disallow empty list as index_spark_colum_names and index_names.

[ueshin]

When an empty list is passed to IndexFrame.index_spark_column_names or .index_names, currently it passes without any errors and it will cause an unexpected error in other places.
We should handle the empty list the same as None.

[codecov-io]

Codecov Report

Merging #1895 (478b0e5) into master (3237002) will increase coverage by 0.01%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #1895      +/-   ##
==========================================
+ Coverage   94.20%   94.22%   +0.01%     
==========================================
  Files          40       40              
  Lines        9939     9932       -7     
==========================================
- Hits         9363     9358       -5     
+ Misses        576      574       -2     

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	91.47% <11.11%> (+0.28%)	⬆️
databricks/koalas/frame.py	96.72% <100.00%> (-0.01%)	⬇️
databricks/koalas/internal.py	96.47% <100.00%> (-0.03%)	⬇️

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