Don't build super image if source images are missing

For some internal branches, vendor.img isn't built, so there
is no need to build super image / super split images there.

Test: remove vendor.img and VENDOR/ from target_files_intermediates, then
      run add_img_to_target_files
Fixes: 120634805

Change-Id: I2834a27ce232538f203733c204dd257279c789fc