Fix spec that started to fail under transproc 0.4.2
Fix spec that started to fail under transproc 0.4.2

This was previously passing by an accident just because transproc's
map_array function used to turn nils into empty arrays, starting with
transproc 0.4.2 we expect an array in `map_array` function and it should
crash if it wasn't an array

rom-rb/rom,rom-rb/rom,rom-rb/rom