fix some cases where the parent is nil in relayable retractions

app/models/relayable_retraction.rb

@@ -21,6 +21,7 @@ class RelayableRetraction < SignedRetraction
   end
 
   def parent
+    return nil unless self.target.present?
     self.target.parent
   end
 

lib/diaspora/relayable.rb

@@ -27,6 +27,7 @@ module Diaspora
 
     # @return [String]
     def parent_guid
+      return nil unless parent.present?
       self.parent.guid
     end