be a bit stricter

lib/avrogen/avro/types/record/field.ex

@@ -60,7 +60,7 @@ defmodule Avrogen.Avro.Types.Record.Field do
   # The only way to differentiate between "default is nil because there isn't one" and "default is actually null" is to
   # check if the type is also a union with null.
   def has_default?(%__MODULE__{default: nil, type: %Types.Union{} = union}),
-    do: Types.Union.has_member?(union, Types.Primitive.null())
+    do: Types.Union.optional_by_convention?(union)
 
   def has_default?(%__MODULE__{default: nil}), do: false
   def has_default?(%__MODULE__{}), do: true

lib/avrogen/avro/types/union.ex

@@ -15,6 +15,7 @@ defmodule Avrogen.Avro.Types.Union do
   """
 
   alias Avrogen.Avro.Schema
+  alias Avrogen.Avro.Types.Primitive
   use TypedStruct
 
   typedstruct do
@@ -25,6 +26,10 @@ defmodule Avrogen.Avro.Types.Union do
   def parse(value) when is_list(value), do: %__MODULE__{types: Enum.map(value, &Schema.parse/1)}
 
   def has_member?(%__MODULE__{types: types}, type), do: Enum.member?(types, type)
+
+  # A union with null listed first is optional by convention (see moduledoc).
+  def optional_by_convention?(%__MODULE__{types: [first_type | _]}),
+    do: first_type == Primitive.null()
 end
 
 alias Avrogen.Avro.Schema.CodeGenerator