Methods browser, platform and device can return bool not only string

[pcmanik]

Methods browser, platform and device can return bool not only string, because method called inside (findDetectionRulesAgainstUA) can return false. They should be updated to return null or string and not false or string. Or at least doc comment must be updated.

https://github.com/jenssegers/agent/blob/bcb895395e460478e101f41cdab139c48dc721ce/src/Agent.php#L222 https://github.com/jenssegers/agent/blob/bcb895395e460478e101f41cdab139c48dc721ce/src/Agent.php#L232 https://github.com/jenssegers/agent/blob/bcb895395e460478e101f41cdab139c48dc721ce/src/Agent.php#L242