The Legacy SDK cannot handle missing strings from objects in sets, and
will insert an empty string when planning the missing value. This
subverts the `couldHaveUnknownBlockPlaceholder` check, and causes
errors when `dynamic` is used with NestingSet blocks.
We don't have a separate codepath to handle the internals of
AssertObjectCompatible differently for the legacy SDK, but we can treat
empty strings as null strings within set objects to avoid the failed
assertions.
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