Use join to concat the both side of the AST

Onf of the sides can be nil and it will raise a Conversion error

actionpack/lib/action_dispatch/journey/visitors.rb

@@ -110,10 +110,11 @@ def visit(node, optional = false)
           def visit_CAT(node, optional)
             left = visit(node.left, optional)
             right = visit(node.right, optional)
+
             if optional && !(right && left)
               ""
             else
-              left +  right
+              [left, right].join
             end
           end