fix(react-storage): handle empty string prefix in controlled mode

[osama-rizk]

Description of changes

Fix StorageBrowser controlled mode validation to properly handle empty string prefix. The validation logic was incorrectly using a falsy check (!location[key]) which treated empty strings as missing parameters.

Issue Reference

• Links to GitHub issue #6807

Validation Process

1. Unit Tests - All pass + new test cases added
2. Build Verification - Successful builds and TypeScript compilation
3. Link the UI package to a sample app and verify the issue is fixed.

Checklist

• Have read the Pull Request Guidelines
• PR description included
• yarn test passes and tests are updated/added
• PR title and commit messages follow conventional commit syntax
• If this change should result in a version bump, changeset added (This can be done after creating the PR.) This does not apply to changes made to docs, e2e, examples, or other private packages.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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Latest commit: 856f83d

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