Shapley contribution when coalition is 0
In the blog post, the author defines the characteristic function $v$ to be the sum of conversion across all subsets of the coalition.  (If I'm understanding the raw data correctly, this is the number of conversions that would've occurred if you limited the channels to that coalition, which seems a sensible measure.)  You can check that in the code definition for v_function.  So in your example, while no customers converted after hitting all four channels, still $v(C_1, C_2, C_3, C_4)\geq 30$, since it includes the sum of the two other channel combinations you've listed.