Fix missing feature name bug during metaproduct generation

In the metaproduct, some method names include the names of the feature, the methods are originally defined in. These feature names are set incorrectly to `null`. The linked screenshot shows the bug with the metaproduct for BankAccount SPL from http://spl2go.cs.ovgu.de/projects/54. This change fixes this bug.
Edit: Screenshot upload does not seem to be working -> https://www.dropbox.com/s/2zku3ncbp7wnmkx/FeatureHouseMetaproductBug.png?dl=0