exekall: Fix ValueDB.merge() on empty list
exekall: Fix ValueDB.merge() on empty list

Return an empty ValueDB, which allows calling code to assume a ValueDB
will always be created.

Also make the None adaptor compatible with any other adaptor, so
everything will work as expected.

credp/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,credp/lisa