chore(growth): return nil when measure isn't set

lib/growth/calc/centile.ex

@@ -2,9 +2,9 @@ defmodule Growth.Calc.Centile do
   @moduledoc """
   Calculate percentile for a given measurement and the fitted values of Box-Cox by age/height:
 
-  * power
-  * median
-  * coefficientof variation
+  * **power** `t:Growth.Calc.ZScore.l/0`
+  * **median** `t:Growth.Calc.ZScore.m/0`
+  * **coefficientof variation** `t:Growth.Calc.ZScore.s/0`
 
   This calculation is described in the [instructions provided by the World Health Organization](https://cdn.who.int/media/docs/default-source/child-growth/growth-reference-5-19-years/computation.pdf).
 
@@ -23,7 +23,7 @@ defmodule Growth.Calc.Centile do
 
   alias Growth.Calc.ZScore
 
-  @spec compute(number(), number(), number(), number()) :: number() | :na
+  @spec compute(number(), ZScore.l(), ZScore.m(), ZScore.s()) :: number() | :na | nil
   @doc """
   Compute the centile for a given measurement (`y`) and the Box-Cox values: power (`l`), median (`m`), and
   coefficient of variation (`s`).
@@ -36,6 +36,10 @@ 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)