pipeline: Ensure timeline state to be NULL on dispose

The GESTimeline's state might not be synced with parent
8 jobs for handle-state-change-failure in 160 minutes and 8 seconds (queued for 62 minutes and 57 seconds)
Status Job ID Name Coverage
  Preparation
passed #76580
gst indent

00:00:17

passed #76579
manifest

00:00:20

 
  Build
passed #76581
build fedora x86_64

00:03:57

 
  Test
passed #76582
check fedora

00:17:08

 
  Full Builds
passed #76583
build android arm64 api28

00:02:36

passed #76585
build cerbero cross win32

00:45:41

passed #76586
build cerbero cross win64

00:43:21

passed #76584
build cerbero fedora x86_64

00:46:43