The "Set Default Value" option in the AttributeManager only works for new attributes.
That forces me to use a NullAttributeMapper for each different default value (multiple attributes).
Adding this to the AttributeManager (Set a default value for Null and Empty attributes) would reduce the number of transformers in my workspace.
Just found out that this can be done in the AttributeManager using conditional values at the Set Value option ( If attributes not has a value, set to new value).