encoding/xml: unmarshal allow empty, non-string values

encoding/xml: unmarshal allow empty, non-string values

When unmarshaling, if an element is empty, eg. '<tag></tag>', and
destination type is int, uint, float or bool, do not attempt to parse
value (""). Set to its zero value instead.

Fixes #13417

Change-Id: I2d79f6d8f39192bb277b1a9129727d5abbb2dd1f
Reviewed-on: https://go-review.googlesource.com/38386
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>