setting a variable with a null value in context cause the application to fail while trying to retrieve it from a (Groovy) Script :
Caused by: groovy.lang.MissingPropertyException: No such property: XXX

The bug is related to the EnvironmentBindings class :
'containsKey(Object key)' method makes use of the method 'get(Object key)' to determine if a Variable exists, but 'get(Object key)' returns the value of the key...

Regards