ExternalProject: Avoid adding empty touch command on "ALWAYS" steps

The problem fixed by commit c0b749cf62 (ExternalProject: Always add a
command to a step, 2014-11-05, v3.2.0-rc1~402^2~3) may occur for an
empty `COMMAND ${touch}` too, so avoid specifying any `COMMAND` at all
if there is no touch command.