Fix ROOT-9784. Avoid to use empty name in v5 'replacement name'.

The mechanism attempting to find v6 equivalent for names found
in v5 files (eg. adding missing namespaces) under some circunstances
(see ROOT-9784) would create class name with missing template parameter
(eg literally map<,RooExpensiveObjectCache::ExpensiveObject*>

This is a fix for commit c412ad28