chore(growth): move nil logic to scorer behaviour

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

lib/growth/score/scorer.ex

@@ -25,7 +25,7 @@ defmodule Growth.Score.Scorer do
       growth
       |> lms(indicator)
       |> Enum.map(fn {precision, {l, m, s}} ->
-        {precision, {z_score(indicator, growth, l, m, s), centile(indicator, growth, l, m, s)}}
+        {precision, scores(indicator, growth, l, m, s)}
       end)
 
     %{growth | results: [Map.new([{indicator.measure_name(), result}]) | growth.results]}
@@ -58,23 +58,41 @@ defmodule Growth.Score.Scorer do
     |> Enum.reject(&is_nil/1)
   end
 
-  @spec z_score(module(), Growth.t(), ZScore.l(), ZScore.m(), ZScore.s()) :: number()
+  @spec scores(module(), Growth.t(), ZScore.l(), ZScore.m(), ZScore.s()) ::
+          {Growth.measure(), number() | :na, nil}
+  @doc """
+  Calculate the z-score and centile of an indicator measurement.
+  """
+  def scores(indicator, growth, l, m, s) do
+    measure = Map.get(growth, indicator.measure_name())
+
+    {
+      z_score(measure, l, m, s),
+      centile(measure, l, m, s)
+    }
+  end
+
+  @spec z_score(Growth.measure(), ZScore.l(), ZScore.m(), ZScore.s()) :: Growth.measure()
   @doc """
   Check `Growth.Calc.ZScore.compute/4`.
   """
-  def z_score(indicator, growth, l, m, s) do
-    growth
-    |> Map.get(indicator.measure_name())
-    |> ZScore.compute(l, m, s)
+  def z_score(nil, _, _, _) do
+    nil
   end
 
-  @spec centile(module(), Growth.t(), ZScore.l(), ZScore.m(), ZScore.s()) :: number() | :na
+  def z_score(value, l, m, s) do
+    ZScore.compute(value, l, m, s)
+  end
+
+  @spec centile(Growth.measure(), ZScore.l(), ZScore.m(), ZScore.s()) :: number() | :na | nil
   @doc """
   Check `Growth.Calc.Centile.compute/4`.
   """
-  def centile(indicator, growth, l, m, s) do
-    growth
-    |> Map.get(indicator.measure_name())
-    |> Centile.compute(l, m, s)
+  def centile(nil, _, _, _) do
+    nil
+  end
+
+  def centile(value, l, m, s) do
+    Centile.compute(value, l, m, s)
   end
 end