Fix simple queries with empty responses

lib/postgrex/protocol.ex

@@ -1253,6 +1253,10 @@ defmodule Postgrex.Protocol do
         {result, s} = done(s, columns, Enum.reverse(rows), [tag])
         recv_simple(s, status, [result | results], [], [], buffer, timeout)
 
+      {:ok, msg_empty_query(), buffer} ->
+        {result, s} = done_empty(s)
+        recv_simple(s, status, [result | results], [], [], buffer, timeout)
+
       {:ok, msg_error(fields: fields), buffer} ->
         err = Postgrex.Error.exception(postgres: fields)
         error_ready(s, status, err, buffer)
@@ -3189,6 +3193,19 @@ defmodule Postgrex.Protocol do
     {result, %{s | messages: []}}
   end
 
+  defp done_empty(%{connection_id: connection_id, messages: messages} = s) do
+    result = %Postgrex.Result{
+      command: nil,
+      num_rows: 0,
+      rows: nil,
+      columns: nil,
+      connection_id: connection_id,
+      messages: messages
+    }
+
+    {result, %{s | messages: []}}
+  end
+
   defp halt(s, query, rows, tag) do
     case done(s, query, rows, tag) do
       {%Postgrex.Result{rows: rows} = result, s} when is_list(rows) ->

test/simple_connection_test.exs

@@ -98,6 +98,24 @@ defmodule SimpleConnectionTest do
       assert result2.num_rows == 1
     end
 
+    test "relaying empty query results", context do
+      assert {:ok,
+              [
+                %Postgrex.Result{
+                  command: nil,
+                  columns: nil,
+                  rows: nil,
+                  num_rows: 0
+                }
+              ]} = SC.call(context.conn, {:query, ""})
+
+      assert {:ok, [%Postgrex.Result{command: nil, rows: nil, num_rows: 0}]} =
+               SC.call(context.conn, {:query, "-- no statement"})
+
+      assert {:ok, [%Postgrex.Result{rows: [["1"]]}]} =
+               SC.call(context.conn, {:query, "SELECT 1"})
+    end
+
     test "relaying query errors", context do
       assert {:ok, %Postgrex.Error{}} = SC.call(context.conn, {:query, "SELCT"})
     end