encoding/xml: prevent omitempty from omitting non-nil pointers to empty values

encoding/xml: prevent omitempty from omitting non-nil pointers to empty values

There was an inconsistency between the (json encoding + documentation)
and the xml encoding implementation. Pointer to an empty value was
not being serialized (i.e simply ignored). Which had the effect of making
impossible to have a struct with a string field for which we wanted to
serialize the value ""

Fixes #5452

Change-Id: Id858701801158409be01e962d2cda843424bd22a
Reviewed-on: https://go-review.googlesource.com/15684
Reviewed-by: Russ Cox <rsc@golang.org>