fix tables unable to sort on nil dates

lib/cannery/comparable_datetime.ex

@@ -0,0 +1,15 @@
+defmodule Cannery.ComparableDateTime do
+  @moduledoc """
+  A custom `DateTime` module that provides a `compare/2` function that is
+  comparable with nil values
+  """
+
+  @spec compare(DateTime.t() | any(), DateTime.t() | any()) :: :lt | :gt | :eq
+  def compare(%DateTime{} = datetime_1, %DateTime{} = datetime_2) do
+    DateTime.compare(datetime_1, datetime_2)
+  end
+
+  def compare(%DateTime{}, _datetime_2), do: :lt
+  def compare(_datetime_1, %DateTime{}), do: :gt
+  def compare(_datetime_1, _datetime_2), do: :eq
+end