Liquid exceptions in builds ('tainted?' method)

See e.g. https://codebase.helmholtz.cloud/hifis/overall/hifis.net/-/jobs/825161#L5322

 Liquid Exception: undefined method `tainted?' for "/assets/img/":String in /builds/hifis/overall/hifis.net/_posts/2019/10/2019-10-29-Blog-Post-Features.md

not sure from first glimpse where this comes from...

@hueser93 @hueser93 could you please have a look?