fix(database): ensure pk_constraint is JSON serializable

[villebro]

SUMMARY

Some databases return sets in the dict payload (they should be lists according to the docs) when calling inspector.get_pk_constraint which doesn't serialize to JSON. In addition, an empty set evaluates to truthy vs lists that evaluate to falsy, causing trouble downstream. By normalizing to JSON serializable datatypes we ensure that the response for the external metadata endpoint will return a valid response.

TEST PLAN

Local testing

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #13147 (4df135c) into master (2ce7982) will increase coverage by 0.38%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master   #13147      +/-   ##
==========================================
+ Coverage   53.06%   53.44%   +0.38%     
==========================================
  Files         489      494       +5     
  Lines       17314    17382      +68     
  Branches     4482     4512      +30     
==========================================
+ Hits         9187     9290     +103     
+ Misses       8127     8092      -35     

Flag	Coverage Δ
cypress	53.44% <63.63%> (+0.38%)	⬆️

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

Impacted Files	Coverage Δ
...end/src/SqlLab/components/RunQueryActionButton.tsx	52.77% <ø> (ø)
superset-frontend/src/chart/ChartContainer.jsx	100.00% <ø> (ø)
superset-frontend/src/chart/ChartRenderer.jsx	75.67% <0.00%> (-1.04%)	⬇️
...perset-frontend/src/common/components/Dropdown.tsx	50.00% <ø> (ø)
...rontend/src/components/ListView/CardSortSelect.tsx	78.94% <ø> (ø)
...ontend/src/components/ListViewCard/ImageLoader.tsx	75.00% <0.00%> (ø)
...set-frontend/src/components/URLShortLinkButton.jsx	100.00% <ø> (ø)
...rset-frontend/src/components/URLShortLinkModal.tsx	77.77% <ø> (ø)
...src/dashboard/components/FiltersBadge/selectors.ts	90.00% <ø> (ø)
...src/dashboard/components/HeaderActionsDropdown.jsx	68.49% <ø> (+0.92%)	⬆️
... and 70 more

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