changed masking such that it works also if belief state has less keys… (#105)

* changed masking such that it works also if belief state has less keys then default state

* Update vector_base.py

Co-authored-by: Christian <christian.geishauser@hhu.de>
Co-authored-by: Carel van Niekerk <40663106+carelvniekerk@users.noreply.github.com>