Fix exception when trying to serialize posts with <a> tags in them without hrefs (#1334)

* fix exception when trying to serialize posts with <a> tags in them without hrefs

* Add tests

Co-authored-by: Thibaut Girka <thib@sitedethib.com>

app/lib/sanitize_config.rb

@@ -55,7 +55,7 @@ class Sanitize
     end
 
     LINK_REL_TRANSFORMER = lambda do |env|
-      return unless env[:node_name] == 'a'
+      return unless env[:node_name] == 'a' and env[:node]['href']
 
       node = env[:node]