Fix undefined protocol error

lib/mox.ex

@@ -707,6 +707,8 @@ defmodule Mox do
     end
   end
 
+  defp add_expectations!(_mock, []), do: :ok
+
   defp add_expectations!(mock, key_expectation_list) do
     validate_mock!(mock)
 

test/mox_test.exs

@@ -835,6 +835,18 @@ defmodule MoxTest do
         assert SciCalcMock.exponent(2, 10) == 1024
       end)
     end
+
+    test "does not crash verify when all callbacks are optional and skipped" do
+      defmodule AllOptionalImpl do
+        @behaviour AllOptionalCallbacks
+        def optional_fun, do: :ok
+      end
+
+      in_all_modes(fn ->
+        stub_with(AllOptionalMock, AllOptionalImpl)
+        verify!()
+      end)
+    end
   end
 
   describe "allow/3" do

test/support/behaviours.ex

@@ -8,3 +8,8 @@ defmodule ScientificCalculator do
   @callback sin(integer()) :: float()
   @optional_callbacks [sin: 1]
 end
+
+defmodule AllOptionalCallbacks do
+  @callback optional_fun() :: :ok
+  @optional_callbacks [optional_fun: 0]
+end

test/support/mocks.ex

@@ -7,6 +7,11 @@ Mox.defmock(SciCalcMockWithoutOptional,
   skip_optional_callbacks: true
 )
 
+Mox.defmock(AllOptionalMock,
+  for: AllOptionalCallbacks,
+  skip_optional_callbacks: true
+)
+
 Mox.defmock(MyMockWithoutModuledoc, for: Calculator)
 Mox.defmock(MyMockWithFalseModuledoc, for: Calculator, moduledoc: false)
 Mox.defmock(MyMockWithStringModuledoc, for: Calculator, moduledoc: "hello world")