meson: Don't generate doc cache when no plugins are enabled

Fixes gst-build with -Dauto-features=disabled
20 jobs for fix-autofeatures-docs in 100 minutes and 58 seconds (queued for 58 seconds)
Status Job ID Name Coverage
  Preparation
passed #428029
gst indent

00:00:47

passed #428028
manifest

00:00:43

 
  Build
passed #428035
build cerbero android universal

00:16:02

passed #428036
build cerbero cross win32

00:09:01

passed #428037
build cerbero cross win64

00:07:09

passed #428034
build cerbero fedora x86_64

00:05:16

manual #428039
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #428038
gst-macos-10.14
build cerbero macos x86_64

00:08:02

passed #428030
build fedora x86_64

00:08:22

passed #428031
build nodebug fedora x86_64

00:06:13

passed #428032
build static fedora x86_64

00:12:44

passed #428033
build static nodebug fedora x86_64

00:09:53

 
  Test
passed #428450
check fedora

00:05:10

passed #428041
integration testsuites fedora

00:13:39

passed #428043
valgrind bad

00:08:45

passed #428044
valgrind ges

00:07:11

passed #428042
valgrind good

00:10:57

failed #428040
check fedora

00:04:18

 
  Integrate
passed #428045
android universal examples

00:07:13

passed #428046
documentation

00:04:21