Fix signature expansion when no docs (#10971)

We were returning an error whenever there wasn't a list of docs. This
changes it so it returns an empty list and the expansion can proceed.

Author: v0idpwn

lib/iex/lib/iex/autocomplete.ex

@@ -128,7 +128,7 @@ defmodule IEx.Autocomplete do
     with docs when is_list(docs) <- get_docs(module, [:function, :macro], name) do
       Enum.map(docs, fn {_, _, signatures, _, _} -> Enum.join(signatures, " ") end)
     else
-      _ -> :error
+      _ -> []
     end
   end
 

lib/iex/test/iex/autocomplete_test.exs

@@ -398,5 +398,11 @@ defmodule IEx.AutocompleteTest do
 
     assert expand('take(') == {:yes, '', ['take(enumerable, amount)']}
     assert expand('derive(') == {:yes, '', ['derive(protocol, module, options \\\\ [])']}
+
+    defmodule NoDocs do
+      def sample(a), do: a
+    end
+
+    assert {:yes, [], [_ | _]} = expand('NoDocs.sample(')
   end
 end