[ci] Fix ti testing with no supported arch. (#3236)
[ci] Fix ti testing with no supported arch. (#3236)

`ti test -vr2 -a vulkan` fails with `TypeError: 'NoneType' object is not
callable` and this PR fixes it.
yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi