Handle facility tags without xlink:href

[krisbukovi]

Some articles have facilities but are missing the xlink:href field which results in "None", see 2016PASP..128e4201C for example.

Facility tags are expected to have the following structure:

<<named-content content-type="facility xlink:href="facilityid">
facility name
</named-content>

however, it is now apparent that there are at least 48 instances where the xlink:href is missing. Per Sergi, we need to keep extracting xlink:href as we are doing now but modify it so that if xlink:href is empty or does not exist (see note below), then we store nothing instead of None (as if the article did not have any facilities).

note: xpath returns None when xlink:href does not exist, but if it is empty xlink:href="" it will return an empty string, so we need to make sure to handle both cases.