Import external "void" from yadmark #17

At the moment, "void" external (i.e. just run myself, leave external empty) is only implemented inside yadmark:

https://github.com/N3PDF/yadism/blob/593a012dc7f4666c2c993dbdd37a6a2d55db0455/benchmarks/yadmark/benchmark/runner.py#L100-L112

This is rather useful (especially thinking about ekobox and yadbox), so I propose:

• move that code from yadmark to banana
• replace "void" (meaningful, but a bit random) with just None (it seems natural to me that, if the external is empty, then no external should be used)