Backwards compatibility for string refs on WWW (#28826)

Seeing errors with undefined string ref values when trying to sync
https://github.com/facebook/react/pull/28473

Added a test that reproduces the failing pattern.

@acdlite pushed
https://github.com/facebook/react/pull/28826/commits/a786481ae5702f1966ecdb62f3667f3d72966e78
with fix

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Co-authored-by: Jack Pope <[email protected]>
Co-authored-by: Andrew Clark <[email protected]>