[henshin-dev] Attribute default value "null" check

[Johannes Tietje <johannes.tietje@xxxxxxxxxxxxxxxxx>]

Hello,

I recently created a set of Henshin rules operating on a Ecore model
with an object (Transition) whose attributes (action, trigger) are of
type String and have a default value of "null". 

How do I specify that an objects attribute is assigned with "null"
without receiving a NullPointerException (which comes up when the rule
matcher tries to match the LHS to the model, i guess)?

Kind regards,
Johannes