减少案例环境和 `\Rightarrow` 之间的水平空间

Question

\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}

\[
\sum\vec{F}= 0 \Rightarrow 
\begin{cases}
\frac{{\partial {\sigma _{xx}}}}{{\partial x}} + \frac{{\partial {\sigma _{yx}}}}{{\partial y}} + \frac{{\partial {\sigma _{zx}}}}{{\partial z}} + {F_x} = 0\\
\frac{{\partial {\sigma _{xy}}}}{{\partial x}} + \frac{{\partial {\sigma _{yy}}}}{{\partial y}} + \frac{{\partial {\sigma _{zy}}}}{{\partial z}} + {F_y} = 0\\
\frac{{\partial {\sigma _{xz}}}}{{\partial x}} + \frac{{\partial {\sigma _{yz}}}}{{\partial y}} + \frac{{\partial {\sigma _{zz}}}}{{\partial z}} + {F_z} = 0
\end{cases}\Rightarrow {\frac{{\partial {\sigma _{ji}}}}{{\partial {x_j}}} + {F_j} = 0}
\]

\end{document}

给出

人们看到案例环境和第二个环境之间存在不受欢迎的空间\Rightarrow。我做错了什么？