While parsing check for existance of attribute

When parsing rel attributes, check for existance of the href attribute
before extracting the text, sometimes like in this post somewhere
https://www.patreon.com/posts/jeenas-excellent-6467133 it might be empty
and causes an exception. All the other uses above this one check that too.

lib/microformats2/collection.rb

@@ -103,6 +103,7 @@ module Microformats2
         else
           rel_values.each do |rel_value|
             @rels[rel_value] = [] unless @rels.has_key?(rel_value)
+            unless rel.attribute("href").nil?
               @rels[rel_value] << Microformats2::AbsoluteUri.new(@base, rel.attribute("href").text).absolutize
             end
           end
@@ -110,3 +111,4 @@ module Microformats2
       end
     end
   end
+end