[FIX] onchange_helper: invert condition to avoid [None]

Co-Authored-By: lmignon <laurent.mignon@acsone.eu>

onchange_helper/models/models.py

@@ -132,7 +132,10 @@ class Base(models.AbstractModel):
         if self:
             self.ensure_one()
 
-        if not isinstance(onchange_fields, list):
+        if not onchange_fields:
+            onchange_fields = values.keys()
+
+        elif not isinstance(onchange_fields, list):
             onchange_fields = [onchange_fields]
 
         if not onchange_fields: