[Backport 2.x] Make sure customer always have a default datasource

[opensearch-trigger-bot]

Backport 91a0530 from #6237.

[codecov]

Codecov Report

Attention: Patch coverage is 82.75862% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 67.28%. Comparing base (bae6984) to head (ba410c7).

Files	Patch %	Lines
...c/components/edit_data_source/edit_data_source.tsx	57.14%	2 Missing and 1 partial ⚠️
...components/data_source_table/data_source_table.tsx	87.50%	1 Missing ⚠️
src/plugins/data_source_management/public/mocks.ts	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x    #6279   +/-   ##
=======================================
  Coverage   67.28%   67.28%           
=======================================
  Files        3341     3341           
  Lines       64682    64711   +29     
  Branches    10354    10360    +6     
=======================================
+ Hits        43519    43543   +24     
- Misses      18649    18652    +3     
- Partials     2514     2516    +2     

Flag	Coverage Δ
Linux_1	31.30% <ø> (ø)
Linux_2	55.39% <ø> (ø)
Linux_3	44.91% <82.75%> (+0.03%)	⬆️
Linux_4	35.20% <0.00%> (-0.01%)	⬇️
Windows_1	31.33% <ø> (ø)
Windows_2	55.35% <ø> (ø)
Windows_3	44.91% <82.75%> (+0.03%)	⬆️
Windows_4	35.20% <0.00%> (-0.01%)	⬇️

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