Cho hàm số \(f\left( x \right)\) xác định bởi \(f\left( x \right) = \left\{ \begin{array}{l}\frac{{\sqrt {{x^2} + 1} - 1}}{x}\,\,\,\left( {x \ne 0} \right)\\0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left( {x = 0} \right)\end{array} \right.\). Giá trị \(f'\left( 0 \right)\) bằng: