vala: fix distcheck with c/vala mixed projects Fixes automake bug#11229. * automake.in (lang_vala_finish_target): Return early if the current '_SOURCES' variable does not contain any '.vala' nor '.vapi' source. Otherwise, the vala compiler will be called without arguments, causing an error. * tests/vala-mix.test: Enhance to catch the fixed bug. Copyright-paperwork-exempt: yes Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com> Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>