Fix DiscordGameSDK runtime exceptions

[ezioleq]

Describe the bug
The DiscordGameSDK is failing to initialize or something, when the Discord isn't open in the operating system. This is not causing any in-game trouble or crashing but is generating an error in logs every time the game is ran.

To Reproduce

1. Start the game (in the editor)
2. See the error in the console

Expected behavior
There's no Discord related errors.

Environment

• OS: Arch Linux (6.12.1-arch1-1)

Additional context

ResultException: InternalError
Discord.Discord..ctor (System.Int64 clientId, System.UInt64 flags) (at Assets/Plugins/DiscordGameSDK/Core.cs:1073)
Portaless.DiscordController.Awake () (at Assets/Scripts/DiscordController.cs:30)

Occasionally you can also get a NullRef exception from the DiscordController.

NullReferenceException: Object reference not set to an instance of an object
Portaless.DiscordController.OnApplicationQuit () (at Assets/Scripts/DiscordController.cs:84)