fix: missing base64 decoding in mpr controller

[eschrewe]

Description

• added missing decoding
• added error handling for input data without proper base64 encoding
• solves: Updating an MPR leads to setting the partner material number to the base64 #579

Pre-review checks

Please ensure to do as many of the following checks as possible, before asking for committer review:

• DEPENDENCIES are up-to-date. Dash license tool. Committers can open IP issues for restricted libs.
• Copyright and license header are present on all affected files
• If helm chart has been changed, the chart version has been bumped to either next major, minor or patch level (compared to released chart).

[tom-rm-meyer-ISST]

LGTM, thanks for fixing!