Fix stat functions with no numeric columns.

[ueshin]

Some statistic functions fail if there are no numeric columns.

>>> kdf = ks.DataFrame({"A": pd.date_range("2020-01-01", periods=3), "B": pd.date_range("2021-01-01", periods=3)})
>>> kdf.mean()
Traceback (most recent call last):
...
ValueError: Current DataFrame has more then the given limit 1 rows. Please set 'compute.max_rows' by using 'databricks.koalas.config.set_option' to retrieve to retrieve more than 1 rows. Note that, before changing the 'compute.max_rows', this operation is considerably expensive.

The functions which allow non-numeric columns by default are:

• count
• min
• max

[codecov-io]

Codecov Report

Merging #1967 (9d673d3) into master (b65891d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1967   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files          49       49           
  Lines       10890    10890           
=======================================
+ Hits        10302    10303    +1     
+ Misses        588      587    -1     

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.79% <100.00%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b65891d...9d673d3. Read the comment docs.

[xinrong-meng]

Great refactoring!

[xinrong-meng]

LGTM! Thank you!

[itholic]

LGTM