fix: add default value on nested inputs to avoid console errors

[letsbelopez]

Problem

Nested inputs have an initial value that is undefined. This causes console errors "uncontrolled to controlled component"

Solution

Add a fallback defined value if it's a nested form field.

Additional Notes

Links

Ticket

GitHub issue _____

aws-amplify/amplify-studio#817

Other links

Verification

Manual tests

Automated tests

• Unit tests added/updated
• E2E tests added/updated
• N/A - (provide a reason)
• deferred - (provide GitHub issue for tracking)

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[codecov-commenter]

Codecov Report

Merging #974 (e9e5756) into main (cb4dba4) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main     #974   +/-   ##
=======================================
  Coverage   93.02%   93.02%           
=======================================
  Files         120      120           
  Lines        5293     5293           
  Branches     1578     1578           
=======================================
  Hits         4924     4924           
  Misses        348      348           
  Partials       21       21           

Impacted Files	Coverage Δ
...eact/lib/forms/form-renderer-helper/value-props.ts	97.29% <100.00%> (ø)

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