fixup! erp5_corporate_identity: allow easy customization of variables used in Letter's template

bt5/erp5_corporate_identity/SkinTemplateItem/portal_skins/erp5_corporate_identity/Base_viewAsLetter.py

@@ -76,7 +76,11 @@ else:
   # cut corner to retrieve path to css files
   letter_version = "001"
   letter_reference = letter.getReference()
-  extra_template_kw = letter.Letter_getExtraTemplateVariableDict()
+
+extra_template_variable_method = letter.getTypeBasedMethod('getExtraTemplateVariableDict')
+extra_template_kw = {}
+if extra_template_variable_method:
+  extra_template_kw = extra_template_variable_method()
 
 # overrides for tests
 if override_batch_mode: