encoding/xml: embedded reference to substruct causes XML marshaller to panic on encoding

When encoding a xml attribute is zero value (IsValid == false), we need
a `continue` to jump over the attribute. If not, followed marshalAttr
function will panic.

Fixes: #50164
Change-Id: I42e064558e7becfbf47728b14cbf5c7afa1e8798
Reviewed-on: https://go-review.googlesource.com/c/go/+/385514


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>