Oracle Infinispan buffer fails to serialize unavailable value placeholders

In DBZ-4276 we introduced the unavailable.value.placeholder configuration option. The runtime represents these unavailable values using OracleValueConverters#UNAVAILABLE_VALUE, which is an Object meant to act as a marker. This violates the LogMinerDmlEntryImplAdapter contract which expected that every value in the old/new column values arrays were String types or null, which results in the following error:

// add error

The adapter should be adjusted to translate the marker object to a valid string-based sentinel value for serialization and then reinterpreted upon loading from disk so that the appropriate runtime value is represented and the array can be properly serialized.