NEW YORK — It is not unlikely that the first Net a visitor to the Barclays Center sees is Joe Johnson. He has a prime place in the Nets’ pregame introduction, a swanky production that highlights the team’s introduction to Brooklyn. Hanging in the arena is a photo memorializing his game-winning, buzzer-beating shot against the Pistons in November. The optics of it all have been golden for Johnson. It is the reality that stings.