Check for null-values

[eugenschindler]

Currently, if one fills in a null-value in a model, plaintextgen will give an NPE. The original rationale for not adding null-value checks was a performance argument: adding such checks and a proper build logging might be a performance hog. My proposal is to experiment with such logging to see how much of a performance hog it really is and build in the null-checks if it doesn't incur too much performance penalty.

[eugenschindler]

@klemensschindler please review this issue