HHH-19386 MutationSpecificationImpl#getResultType must return null

Returning `Void.class` causes the validation in `QuerySqmImpl`
to fail with the error:
```
org.hibernate.query.IllegalQueryOperationException: Result type given for a non-SELECT Query [<criteria>]
```

when the query is executed like in the example:
```java
MutationSpecification  mutationSpecification  = MutationSpecification.create( ... )
session
   .createQuery(mutationSpecification.reference())
   .executeUpdate();
```

Author: DavideD

hibernate-core/src/main/java/org/hibernate/query/specification/internal/MutationSpecificationImpl.java

@@ -79,7 +79,7 @@ public String getName() {
 
 	@Override
 	public Class<Void> getResultType() {
-		return Void.class;
+		return null;
 	}
 
 	@Override