Avoid calling objective function's methods in Scaled if multiplier is zero

[santisoler]

If the Scaled.multiplier is zero, then we could avoid calling the underlying method of the objective function that might require some computational load that will be discarded as soon as we multiply it by the scalar multiplier. We could have return a null result instead:

• ___call__: return 0.0.
• gradient: return an array full of zeros.
• hessian: return a sparse matrix with zeros (maybe a diagonal one...)