Data-driven enumerations fail when other enumerations do not exist

*Moved from https://issues.jboss.org/browse/JBRULES-3270*

Currently, the following enumeration requires an enumeration on engineType to work correctly:

'Vehicle.fuelType[engineType]' : '(new test.VehicleHelper()).getFuelTypes("@

")'

Without an enumeration on engineType, the above fuelType enumeration does not work. Drools should not require an enumeration on engineType, as it may not always be needed.