expression_tree_walker() and primitive node types

I wouldn't call this behavior buggy, but I found it somewhat surprising.

expression_tree_walker() assumes that the walker has already been
invoked on the current node (the node that a given recursive call of
expression_tree_walker() has been invoked on). Therefore, calling
expression_tree_walker() on a primitive node type, such as a Var, is a
no-op.