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

Fixes gst-build with -Dauto-features=disabled

GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good