msdkenc: return NULL instead of FALSE

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

00:00:29

passed #76293
manifest

00:00:35

 
  Build
passed #76295
build fedora x86_64

00:02:51

 
  Test
passed #76296
check fedora

00:15:15

 
  Full Builds
passed #76297
build android arm64 api28

00:04:02

passed #76299
build cerbero cross win32

00:37:42

passed #76300
build cerbero cross win64

00:37:50

passed #76298
build cerbero fedora x86_64

00:12:45