chore: allow empty run metadata requests to delete existing metadata

[corban-beaird]

Ticket

ET-307

Description

Enables empty an metadata field to be submitted through both the Core API & API endpoints, allowing existing metadata to be deleted internally.

Cleans up an unnecessary todo from TestFlattenMetadataNil about returning an error when the provided metadata is empty, this no longer was the decided case since the expectation is to delete existing metadata when the provided metadata is empty.

Test Plan

• TestFlattenMetadataNil
• TestFlattenMetadata
• TestPostRunMetadata

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

[netlify]

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Name	Link
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[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 49.28%. Comparing base (b51bc93) to head (28ccb21).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9524   +/-   ##
=======================================
  Coverage   49.28%   49.28%           
=======================================
  Files        1242     1242           
  Lines      161442   161442           
  Branches     2868     2868           
=======================================
  Hits        79570    79570           
  Misses      81700    81700           
  Partials      172      172           

Flag	Coverage Δ
backend	43.90% <33.33%> (ø)
harness	63.81% <ø> (ø)
web	44.87% <ø> (ø)

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

Files	Coverage Δ
master/internal/api_runs.go	63.23% <ø> (+0.03%)	⬆️
master/internal/run/runs.go	87.09% <100.00%> (ø)
master/internal/db/postgres_runs.go	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes