\documentclass[10pt]{article}
\usepackage{empheq}
\usepackage{amsmath, amssymb, color}
\usepackage{xepersian}

\definecolor{shadecolor}{cmyk}{0,0,0.41,0}
\definecolor{light-blue}{cmyk}{0.25,0,0,0}
\newsavebox{\mysaveboxM} % M for math
\newsavebox{\mysaveboxT} % T for text
\renewcommand*\Box[2][]{%
 \sbox{\mysaveboxM}{#2}%
   \sbox{\mysaveboxT}{\fcolorbox{black}{light-blue}{#1}}%
     \sbox{\mysaveboxM}{%
       \parbox[b][\ht\mysaveboxM +0.5\ht\mysaveboxT +0.5\dp\mysaveboxT][b]{%
         \wd\mysaveboxM}{#2}%
          }%
\sbox{\mysaveboxM}{%
  \fcolorbox{black}{shadecolor}{%
   \makebox[\linewidth -10em]{\usebox{\mysaveboxM}}%
    }%
}%
\usebox{\mysaveboxM}%
 \makebox[0pt][l]{%
  \makebox[\wd\mysaveboxM][c]{%
   \raisebox{\ht\mysaveboxM -0.5\ht\mysaveboxT
                                       +0.5\dp\mysaveboxT -0.5\fboxrule}{\usebox{\mysaveboxT}}%
        }%
    }%
 }

\begin{document}
 \begin{empheq}[left={|y|= \empheqlbrace}, box={\Box[متن در وسط خط بالای جدول قرار بگیرد]}]{alignat*=4}
&  y       \qquad              &     y>0\\
&-y     \qquad      &  y<0
\end{empheq}
\end{document}