[Formulator] Field does not create empty TALES expressions

Maybe @kazuhiko  is interested in this since you are using the edit_xmlrpc methods sometimes in your tests, right?

This effectively allows to zero-out some TALES expressions via manage_tales_xmlrpc which was not possible before. It is enough to set them to an empty string. 

/reviewed-on nexedi/erp5!563

product/Formulator/Field.py

@@ -651,7 +651,10 @@ class ZMIField(
         from TALESField import TALESMethod
         result = {}
         for key, value in map.items():
-            result[key] = TALESMethod(value)
+            if value:
+                result[key] = TALESMethod(value)
+            else:
+                result[key] = ''  # do not create empty methods
         self._edit_tales(result)
 
     # display test screen