Fix: useOptimistic should return passthrough value when there are no updates pending (#27936)

This fixes a bug that happened when the canonical value passed to
useOptimistic without an accompanying call to setOptimistic. In this
scenario, useOptimistic should pass through the new canonical value.

I had written tests for the more complicated scenario, where a new value
is passed while there are still pending optimistic updates, but not this
simpler one.