The existing code incorrectly forbid setting a tag on a table to an empty
string value, but this is allowed by DynamoDB and is useful, so we fix it
in this patch.
While at it, improve the error-checking code for tag parameters to
cleanly detect more cases (like missing or non-string keys or values).
The following patch is a test that fails before this patch (because
it fails to insert a tag with an empty value) and passes after it.
Fixes#16904.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>