Bug 960525 - [Contacts] undefined as contact's last name after merging

The problem was due to https://github.com/mozilla-b2g/gaia/blob/master/apps/communications/contacts/js/contacts_merger.js#L123 without checking if there was any truthy value in familyName[0].

I didn't want to include the condition |familyName.length| instead to avoid cases such as [null, "someValue"], etc. I know, not really expected but who knows... :-p