Fixing typing issue with ApiCallback where error or result might be null

[jondewoo]

From PlayFabSdk/Scripts/PlayFab/PlayFab.js:

if (replyEnvelope.hasOwnProperty("error") || !replyEnvelope.hasOwnProperty("data")) {
    callback(replyEnvelope, null);
} else {
    callback(null, replyEnvelope);
}

[jondewoo]

Please consider this PR as the issue is causing errors when strictNullChecks is true in a TypeScript project.