chore: handle empty nested structs in run metadata as nil leaf nodes

[corban-beaird]

Ticket

ET-306

Description

When submitting metadata with nested structs (maps/lists) that contain no elements, this would previously cause a 500 response. Instead, this changes that behavior to now treat these instances as leaf nodes for indexing with nil values & stores the metadata as expected.

Test Plan

• Automated test runs_test.go:TestFlattenMetadataEmptyNestedStructs

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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.29%. Comparing base (b51bc93) to head (ae3e000).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9526   +/-   ##
=======================================
  Coverage   49.28%   49.29%           
=======================================
  Files        1242     1242           
  Lines      161442   161452   +10     
  Branches     2868     2867    -1     
=======================================
+ Hits        79570    79588   +18     
+ Misses      81700    81692    -8     
  Partials      172      172           

Flag	Coverage Δ
backend	43.93% <100.00%> (+0.02%)	⬆️
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/run/runs.go	88.34% <100.00%> (+1.25%)	⬆️

... and 5 files with indirect coverage changes

[AmanuelAaron]

Looks good. Do we know why we don't get this error when there are other metadata types provided?