handle nil configuration returned by workspace/configuration

the type is LSPAny in https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#workspace_configuration
Fixes #1001

apps/language_server/lib/language_server/server.ex

@@ -1467,7 +1467,11 @@ defmodule ElixirLS.LanguageServer.Server do
       response = JsonRpc.get_configuration_request(state.root_uri, "elixirLS")
 
       case response do
-        {:ok, [result]} when is_map(result) ->
+        {:ok, [result]} when is_map(result) or is_nil(result) ->
+          # result type is LSPAny, we need to handle at least map and nil
+          # https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#workspace_configuration
+          result = result || %{}
+
           Logger.info(
             "Received client configuration via workspace/configuration\n#{inspect(result)}"
           )

apps/language_server/test/server_test.exs

@@ -111,6 +111,39 @@ defmodule ElixirLS.LanguageServer.ServerTest do
       end)
     end
 
+    test "handles nil configuration", %{
+      server: server
+    } do
+      in_fixture(__DIR__, "clean", fn ->
+        Server.receive_packet(
+          server,
+          initialize_req(1, root_uri(), %{
+            "workspace" => %{
+              "configuration" => true
+            }
+          })
+        )
+
+        assert_receive(%{"id" => 1, "result" => %{"capabilities" => %{}}}, 1000)
+        Server.receive_packet(server, notification("initialized"))
+        uri = root_uri()
+
+        assert_receive(
+          %{
+            "id" => 1,
+            "method" => "workspace/configuration",
+            "params" => %{"items" => [%{"scopeUri" => ^uri, "section" => "elixirLS"}]}
+          },
+          1000
+        )
+
+        JsonRpc.receive_packet(response(1, [nil]))
+
+        assert :sys.get_state(server).mix_env == "test"
+        wait_until_compiled(server)
+      end)
+    end
+
     test "gets configuration after workspace/didChangeConfiguration notification if client supports it",
          %{
            server: server