Set value in our empty key value to Tephra won't treat it like a column delete

Tephra interprets an empty column value as a Column delete. We don't want this to be the case for our empty KeyValue. Since we're only allowing a new table to be set as transactional, let's set the value in our empty KeyValue for transactional tables to a single byte. Then we can undo the change I made to force STORE_NULLS to be true for transactional tables (in MetaDataClient). In addition, we can set the TransactionAwareHTable flag to not treat an empty value as a delete if STORE_NULLS is true.