Fix null author on DMs received from other bots

src/main/java/net/dv8tion/jda/api/entities/channel/concrete/PrivateChannel.java

@@ -39,6 +39,7 @@ public interface PrivateChannel extends MessageChannel
      *     <li>A reaction is added</li>
      *     <li>A message is deleted</li>
      *     <li>This account sends a message to a user from another shard (not shard 0)</li>
+     *     <li>This account receives an interaction response, happens when using an user-installed interaction</li>
      * </ul>
      * The consequence of this is that for any message this bot receives from a guild or from other users, the user will not be null.
      *

src/main/java/net/dv8tion/jda/internal/entities/EntityBuilder.java

@@ -1630,11 +1630,10 @@ else if (channel instanceof PrivateChannel)
             }
             else
             {
-                //Note, while PrivateChannel.getUser() can produce null, this invocation of it WILL NOT produce null
-                // because when the bot receives a message in a private channel that was _not authored by the bot_ then
-                // the message had to have come from the user, so that means that we had all the information to build
-                // the channel properly (or fill-in the missing user info of an existing partial channel)
+                //PrivateChannel.getUser() can produce null, see docs
                 user = ((PrivateChannel) channel).getUser();
+                if (user == null)
+                    user = createUser(author);
             }
         }
         else