fix release short name validated failed

[t-feng]

Hello:
I was making Linux distributions based on Fedora(lorax for build, anaconda for installer). But it is failed during anaconda install.
After simply debuging for that, I found it is failed in parsering .treeinfo which is a hidden file in ISO.
Because, release-short-name in my treeinfo is not startwith Fedora, CentOS or RHEL.
So, Type-Check failed...
def _validate_short(self): self._assert_type("short", list(six.string_types))
If deserialize_0_0 return "" other then None, It would be better ;)

[lubomir]

Out of curiosity: what is the name of your distribution?

[lubomir]

As far as I can tell, nothing directly in productmd should break by having an empty string used as short name.
I'd say it's a reasonably safe change - anything that worked until now will continue to do so without changes. Files that were running into this issue will either start working, or maybe the calling code will fail due to not expecting an empty string. That seems to be fine to me.

Thank you for the patch!