[lib] fix pinned content for non-media messages

Summary:
Context in ENG-3897.

To fix problem adding condition if media_infos is empty array (which should prevent calling translateClientDBMediaInfosToMedia for non-media messages.

Test Plan: Check on physical device and prod backend if error disappeared.

Reviewers: tomek, atul

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7842