gst-libav fatal error if plugins_cache_generatorplugins_cache_generator not found
Noticed while doing external build of gstreamer 1.20.0. We probably have packaging issue which exposed this but whenever https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-libav/docs/meson.build#L23 plugins_cache_generator
is not found, plugins_doc_dep
is not defined so https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-libav/docs/meson.build#L77 processes undefined variable.