Handle undefined text area values

Check to see if a text area's `val` is defined before trying to call
`replace()` on it.

app/assets/javascripts/application.js.coffee

@@ -54,7 +54,7 @@ window.extractLast = (term) ->
   return split( term ).pop()
 
 window.rstrip = (val) ->
-  return val.replace(/\s+$/, '')
+  return if val then val.replace(/\s+$/, '') else val
 
 # Disable button if text field is empty
 window.disableButtonIfEmptyField = (field_selector, button_selector) ->