Princess: Handle String Literals in Evaluation of Partial Model

[kfriedberger]

Princess does not report evaluations for String literals in partial models. The issue was reported to the developers in uuverifiers/ostrich#108. Maybe an update of Ostrich helps. The current version 2.0 is still the most recent version, as version 2.0.1 is not yet released into Maven properly.

Thi issue is a follow-up from #528.