Do not set empty avatar when generating one

As the method is called on write and is also calling write via save, we must
ensure to save only when an avatar is filled to break the loop.

issue10914
review363041002

trytond/model/avatar.py

@@ -62,7 +62,9 @@ def avatar_mixin(size=64, default=None):
             if not records:
                 return
             for record in records:
-                record.avatar = generate(size, getattr(record, field))
+                avatar = generate(size, getattr(record, field))
+                if avatar:
+                    record.avatar = avatar
             cls.save(records)
 
         if default: