vala: fix distcheck with c/vala mixed projects

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>