Fix completion crash on module attribute call (#451)

lib/livebook/completion.ex

@@ -106,6 +106,10 @@ defmodule Livebook.Completion do
     {:ok, List.to_atom(var)}
   end
 
+  defp expand_dot_path({:module_attribute, _attribute}, _ctx) do
+    :error
+  end
+
   defp expand_dot_path({:dot, parent, call}, ctx) do
     case expand_dot_path(parent, ctx) do
       {:ok, %{} = map} -> Map.fetch(map, List.to_atom(call))

test/livebook/completion_test.exs

@@ -925,4 +925,9 @@ defmodule Livebook.CompletionTest do
              }
            ] = Completion.get_completion_items("@modu", binding, env)
   end
+
+  test "handles calls on module attribute" do
+    {binding, env} = eval(do: nil)
+    assert [] = Completion.get_completion_items("@attr.value", binding, env)
+  end
 end