Let \( f(x) = \dfrac{\sin x + \cos x - \sqrt{2}}{\sin x - \cos x},\; x \in [0, \pi] - \left\{ \dfrac{\pi}{4} \right\}. \) Then \( f\!\left(\dfrac{7\pi}{12}\right) f''\!\left(\dfrac{7\pi}{12}\right) \) is equal to:
\( \dfrac{2}{9} \)
\( \dfrac{2}{3\sqrt{3}} \)
\( \dfrac{-1}{3\sqrt{3}} \)
\( \dfrac{-2}{3} \)