Fixed EZP-20818: [eZFLow] Adding a source to multimedia carousel throws an missing info error

This is related to the change introduced by fixing EZP-20455:
https://github.com/ezsystems/ezpublish-legacy/commit/e39d2077d6dbf2a2b1de4597733d2f89cbc0f338
(cherry picked from commit 47b736b1fdb05b6f2eaf12b5b97472dc8c4dbcaa)