chore(growth): move nil logic to scorer behaviour

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

lib/growth/calc/centile.ex

@@ -23,7 +23,7 @@ defmodule Growth.Calc.Centile do
 
   alias Growth.Calc.ZScore
 
-  @spec compute(number(), ZScore.l(), ZScore.m(), ZScore.s()) :: number() | :na | nil
+  @spec compute(number(), ZScore.l(), ZScore.m(), ZScore.s()) :: number() | :na
   @doc """
   Compute the centile for a given measurement (`y`) and the Box-Cox values: power (`l`), median (`m`), and
   coefficient of variation (`s`).
@@ -36,10 +36,6 @@ defmodule Growth.Calc.Centile do
       19.0
 
   """
-  def compute(nil, _, _, _) do
-    nil
-  end
-
   def compute(y, l, m, s) do
     zscore = ZScore.raw(y, l, m, s)