I am running this command:
$ java -jar jcov.jar RepGen -o report -i java.util -javap java.base/classes/java/util/OptionalDouble.class merged.xml

RepGen generates an empty report.

$ ls java.base/classes/java/util/Optional.class
java.base/classes/java/util/Optional.class
$ grep '"Optional"' merged.xml
                <class name="Optional" supername="java/lang/Object" checksum="281279722" signature="&lt;T:Ljava/lang/Object;&gt;Ljava/lang/Object;" source="Optional.java" flags=' public final'>