gsturi: Fix annotation on get_path to return a nullable

Use proper syntax or the (nullable): part will be part of the description
9 jobs for annotation-fixes in 34 minutes and 6 seconds (queued for 39 minutes and 45 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #156761
gst indent

00:00:26

passed #156760
manifest

00:00:49

 
  Build
passed #156762
build fedora x86_64

00:04:59

 
  Test
passed #157102
check fedora

00:04:04

failed #156763
check fedora

00:04:03

 
  Full Builds
passed #156764
build android arm64 api28

00:03:39

passed #156766
build cerbero cross win32

00:07:49

passed #156767
build cerbero cross win64

00:10:15

passed #156765
build cerbero fedora x86_64

00:07:04