chore(growth): move nil logic to scorer behaviour

Handle `nil` measurement in `Growth.Score.Scorer`.

lib/growth/calc/z_score.ex

@@ -34,7 +34,7 @@ defmodule Growth.Calc.ZScore do
   """
   @type s :: number()
 
-  @spec compute(number(), l(), m(), s()) :: Growth.measure()
+  @spec compute(number(), l(), m(), s()) :: number()
   @doc """
   Compute the adjusted z-score for a given measurement (`y`) and the Box-Cox values: power (`l`), median (`m`), and
   coefficient of variation (`s`).
@@ -49,11 +49,6 @@ defmodule Growth.Calc.ZScore do
       1.4698319520484722
 
   """
-
-  def compute(nil, _, _, _) do
-    nil
-  end
-
   def compute(y, l, m, s) do
     y
     |> raw(l, m, s)