vala: Keep depfile empty if no deps were written
Workaround for https://github.com/ninja-build/ninja/issues/2357.
Context: I'm testing use of depfiles with valac in meson and two of its test cases are hitting this problem.
Workaround for https://github.com/ninja-build/ninja/issues/2357.
Context: I'm testing use of depfiles with valac in meson and two of its test cases are hitting this problem.