Added userID guard

[Revadike]

Because I got this:

 2019-06-12 | 01:22:30.151 | ERROR | crashBot                       | Fatal error: TypeError: Cannot read property 'getSteamID64' of null
    at get (node_modules\steamcommunity\components\users.js:416:60)
    at SteamCommunity.getUserInventoryContents (node_modules\steamcommunity\components\users.js:412:2)
    at unpackSteamBoosterPacks (index.js:3588:21)
    at cleanupSteamInventory (index.js:3583:5)
    at SteamUser.refreshSteamWebSession (index.js:857:38)
    at Object.onceWrapper (events.js:273:13)
    at SteamUser.emit (events.js:182:13)
    at SteamUser.gotSteamWebSession (index.js:1754:16)
    at SteamUser.emit (events.js:182:13)
    at _apiRequest (node_modules\steam-user\components\web.js:61:9)
    at IncomingMessage.<anonymous> (node_modules\steam-user\components\webapi.js:82:4)
    at IncomingMessage.emit (events.js:187:15)
    at endReadableNT (_stream_readable.js:1094:12)
    at process._tickCallback (internal/process/next_tick.js:63:19)