fix(Dashboard): Save empty label_colors in json_metadata

[geido]

SUMMARY

It was impossible to delete the entire label_colors object in json_metadata as the backend would not update it. This PR fixes that

TESTING INSTRUCTIONS

1. Open a Dashboard with custom label_colors in json_metadata (Edit properties)
2. Delete the entire label_colors object
3. Refresh the Dashboard
4. Make sure the label_colors are deleted

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 #17462 (645ab4f) into master (c54027a) will decrease coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #17462      +/-   ##
==========================================
- Coverage   76.94%   76.72%   -0.23%     
==========================================
  Files        1042     1042              
  Lines       56299    56311      +12     
  Branches     7793     7793              
==========================================
- Hits        43322    43207     -115     
- Misses      12721    12848     +127     
  Partials      256      256              

Flag	Coverage Δ
hive	?
mysql	81.94% <100.00%> (-0.02%)	⬇️
postgres	81.95% <100.00%> (-0.02%)	⬇️
presto	?
python	82.03% <100.00%> (-0.43%)	⬇️
sqlite	81.62% <100.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
superset/dashboards/dao.py	96.09% <100.00%> (-0.04%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/db_engine_specs/hive.py	69.49% <0.00%> (-16.99%)	⬇️
superset/db_engine_specs/presto.py	83.47% <0.00%> (-6.91%)	⬇️
superset/tasks/async_queries.py	91.02% <0.00%> (-5.95%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/connectors/sqla/models.py	86.79% <0.00%> (-1.59%)	⬇️
superset/models/core.py	89.26% <0.00%> (-0.74%)	⬇️
superset/charts/data/api.py	87.91% <0.00%> (-0.66%)	⬇️
superset/db_engine_specs/base.py	88.20% <0.00%> (-0.39%)	⬇️
... and 2 more

---

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 c54027a...645ab4f. Read the comment docs.

[zhaoyongjie]

LGTM

[villebro]

LGTM - just curious, could the same problem happen with color_namespace?

[geido]

LGTM - just curious, could the same problem happen with color_namespace?

Yes it might be. I have a PR upcoming related to the color scheme. I'll test it and fix it there. Thanks!