设 $f_1(t)=\left\{\begin{array}{ll}0, & t < 0 \\ 1, & t \geq 0\end{array}, \quad f_2(t)=\left\{\begin{array}{ll}0, & t < 0 \\ e^{-t}, & t \geq 0\end{array}\right.\right.$ 求 $f_1(t) * f_2(t)$ 。