Fix dtext.parse_strip failing for nil (closes #2662)

app/logical/d_text.rb

@@ -339,6 +339,8 @@ class DText
   end
 
   def self.strip(s)
+    return "" if s.blank?
+
     s.gsub!(/[\r\n]+/m, " ")
     s.gsub!(/\[\/?(?:b|i|s|u|tn|tr|td|th|thead|tbody|quote|code|spoilers|spoiler|expand|table)\]/, "")
     s.gsub!(/\[\[([^\|\]]+)\|([^\]]+)\]\]/m, '\2')