forms: Clearing answers cache should be null

The check in DynamicFormEntry::getAnswers() will use the cached _values
array if it is not set to null. Previously, the ::save() method would set it
to an empty array.

include/class.dynamic_forms.php

@@ -537,7 +537,7 @@ class DynamicFormEntry extends VerySimpleModel {
             if ($field->hasData() && !$field->isPresentationOnly())
                 $a->save();
         }
-        $this->_values = array();
+        $this->_values = null;
     }
 
     function delete() {