Don't check session data contents if they are nil

[kstenerud]

Goal

When a session is resumed, we send an update notification with a nil object to indicate that there's no new session info to update:

    [center addObserver:self selector:@selector(sessionUpdateNotification:) name:BSGSessionUpdateNotification object:nil];

In our update handler, it fails the JSON check so the data doesn't get updated (as we want), but it also prints out an ominous error message:

if (![BSGJSONSerialization isValidJSONObject:notification.object]) {
    bsg_log_err("Invalid session payload in notification");
    return;
}

This PR does an explicit nil check so that it doesn't print an error message on session resume.

Testing

Reran e2e tests

[github-actions]

Bugsnag.framework binary size did not change - 913,280 bytes

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  -0.2%     -56  -0.2%     -56    [__TEXT]
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[nickdowell]

Tested it locally, LGTM 👍