fix: allow for multiple columns in pivot table report

[eschutho]

SUMMARY

This fixes a 500 error when running a pivot table v2 report on a chart with multiple columns and no rows
On a pandas dataframe stack function:
if the columns have a single level, the output is a Series;
if the columns have multiple levels, the new index level(s) is (are) taken from the prescribed level(s) and the output is a DataFrame.

For a chart with multiple columns, a dataframe does not have the to_frame method, and was erroring.
This pr checks for the return type of stack before calling to_frame

Second fix: some charts that have null values were having problems when pivoting because pandas will drop some data and then later filtering and manipulation can cause errors. I resolved this by filling in null values with a "NULL" string.

*some work still needs to be done to improve the formatting of a pivot chart report with multiple columns.

TESTING INSTRUCTIONS

Create a chart with pivot v2 with multiple columns and no rows and make sure that a chart as both a csv and table can be sent as a report.
Create a pivot v2 chart with a column of all null values and select that as a chart column. Reports should send correctly for csv and test.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #17636 (1e03ebd) into master (7353a2b) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #17636      +/-   ##
==========================================
- Coverage   76.81%   76.73%   -0.08%     
==========================================
  Files        1049     1062      +13     
  Lines       56653    57005     +352     
  Branches     7847     7847              
==========================================
+ Hits        43517    43745     +228     
- Misses      12883    13007     +124     
  Partials      253      253              

Flag	Coverage Δ
mysql	81.80% <100.00%> (-0.21%)	⬇️
postgres	81.81% <100.00%> (-0.21%)	⬇️
python	81.89% <100.00%> (-0.21%)	⬇️
sqlite	81.49% <100.00%> (-0.21%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/charts/post_processing.py	68.21% <100.00%> (+1.01%)	⬆️
superset/utils/cache_manager.py	96.15% <0.00%> (-3.85%)	⬇️
superset/connectors/base/models.py	88.19% <0.00%> (-0.49%)	⬇️
superset/dashboards/api.py	91.86% <0.00%> (-0.29%)	⬇️
superset/charts/commands/export.py	94.11% <0.00%> (-0.17%)	⬇️
superset/cli.py	54.82% <0.00%> (-0.11%)	⬇️
superset/views/core.py	77.38% <0.00%> (-0.06%)	⬇️
superset/db_engine_specs/base.py	88.15% <0.00%> (-0.05%)	⬇️
superset/constants.py	100.00% <0.00%> (ø)
superset/db_engine_specs/hana.py	95.00% <0.00%> (ø)
... and 48 more

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[betodealmeida]

6 lines of code, 583 of tests! <3

[eschutho]

6 lines of code, 583 of tests! <3

Props to you for writing the original tests. :) This code was very easy to work with as a result!