[5.x] Ensure updated_at and updated_by is not null in TracksLastModified

[simonerd]

In one of our customer projects that we finally were allowed to update to v5 we encountered an error when saving certain entries:

Carbon\Carbon::createFromTimestamp(): Argument #1 ($timestamp) must be of type string|int|float, null given, called in /var/www/html/vendor/statamic/cms/src/Data/TracksLastModified.php on line 15

On closer inspection it turned out that the affected entries have updated_at: null in their data. I currently can't quite figure out why that is the case, but this causes a problem in TracksLastModified::lastModified():

public function lastModified()
{
    return $this->has('updated_at')
        ? Carbon::createFromTimestamp($this->get('updated_at'), config('app.timezone'))
        : $this->fileLastModified();
}

The entry has updated_at, but it is null, causing the Carbon error.

This PR simply changes the lastModified() function (and lastModifiedBy() too, for good measure) to not just check if the key is there, but also if its truthy.