Parse empty xml nodes that are strings are an empty string

This allows `get_*` operations to better round trip with the
corresponding `put_*` operations.

We can get the code reviewed, but I'm not sure this is change we want.
It has the potential to break users so we'll need to evaluate if we can take
this change.

cc @kyleknap @danielgtaylor 

Fixes https://github.com/aws/aws-cli/issues/1076