empty attributes are not handled in implied properties

[sknebel]

This is a followup to #118: while there handling for empty attributes as actual properties was handled, this is still an issue for implied properties: http://php.microformats.io/?id=20170603110117771

Most important in url-properties, where this might appear with author h-cards.

Test cases are available here: microformats/tests@43a9173