fix: import metrics with extra

[betodealmeida]

SUMMARY

When importing a dataset, if a metric has extra = {} it doesn't get serialized to a string because it doesn't meet the condition if metric.get("extra"). I fixed it by being explicit and checking if metric.get("extra") is not None.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

Updated unit test to cover this case. Tested with dashboard that was not working before.

ADDITIONAL INFORMATION

• Has associated issue:
• 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 #15047 (e819ced) into master (b75df93) will increase coverage by 0.23%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #15047      +/-   ##
==========================================
+ Coverage   77.55%   77.78%   +0.23%     
==========================================
  Files         966      966              
  Lines       49624    49628       +4     
  Branches     6314     6314              
==========================================
+ Hits        38485    38605     +120     
+ Misses      10938    10822     -116     
  Partials      201      201              

Flag	Coverage Δ
hive	81.42% <50.00%> (?)
mysql	81.70% <50.00%> (-0.01%)	⬇️
postgres	81.73% <50.00%> (-0.01%)	⬇️
presto	81.42% <50.00%> (?)
python	82.25% <50.00%> (+0.44%)	⬆️
sqlite	81.34% <50.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
superset/datasets/commands/importers/v1/utils.py	58.97% <50.00%> (+1.83%)	⬆️
superset/utils/core.py	89.04% <0.00%> (+0.12%)	⬆️
superset/utils/mock_data.py	25.18% <0.00%> (+0.18%)	⬆️
superset/models/core.py	90.00% <0.00%> (+0.26%)	⬆️
superset/db_engine_specs/base.py	88.37% <0.00%> (+0.40%)	⬆️
superset/connectors/sqla/models.py	89.81% <0.00%> (+1.65%)	⬆️
superset/views/database/mixins.py	82.75% <0.00%> (+1.72%)	⬆️
superset/db_engine_specs/presto.py	90.31% <0.00%> (+6.94%)	⬆️
superset/db_engine_specs/hive.py	87.39% <0.00%> (+17.07%)	⬆️
superset/db_engines/hive.py	82.14% <0.00%> (+82.14%)	⬆️

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