Avoid NPE in TableViews.allViewsCfs when a table has no views

src/java/org/apache/cassandra/db/view/TableViews.java

@@ -89,6 +89,9 @@ public class TableViews extends AbstractCollection<View>
 
     public Iterable<ColumnFamilyStore> allViewsCfs()
     {
+        if (views.isEmpty())
+            return Collections.emptyList();
+
         Keyspace keyspace = Keyspace.open(baseTableMetadata.keyspace);
         return Iterables.transform(views, view -> keyspace.getColumnFamilyStore(view.getDefinition().name()));
     }