msdkenc: return NULL instead of FALSE

The returned type is a pointer
8 jobs for fix-pool in 56 minutes and 34 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Preparation
passed gst indent #76294

00:00:29

passed manifest #76293

00:00:35

 
  Build
passed build fedora x86_64 #76295

00:02:51

 
  Test
passed check fedora #76296

00:15:15

 
  Full Builds
passed build android arm64 api28 #76297

00:04:02

passed build cerbero cross win32 #76299

00:37:42

passed build cerbero cross win64 #76300

00:37:50

passed build cerbero fedora x86_64 #76298

00:12:45