Spec needs update - Interpretation of empty values for different primitive types.

Previously tracked via: https://bugs.openjdk.java.net/browse/JDK-8170796

JAXB spec does not explicitly state what must be the behavior of unmarshalling implementation, when unmarshaling is attempted on an element with empty value for any specific type.

Review the JAXB specification and the previously tracked JDK-8170796 issue to determine the intended behavior for unmarshalling empty values of each primitive type. Done means the specification explicitly defines that behavior; the payload does not identify a specification file or test to run.

Written by the indexing model from the issue text.