How to not contain Title-Name in table-of-contents at the sidebar of the document? (Berkeley theme)

Since the outer theme sidebar that is used by the theme Berkley inserts the short versions of author and title, you can simply leave them blank:

documentclass[9pt]{beamer}
usetheme{Berkeley}
usepackage[utf8]{inputenc}
usepackage[english,greek]{babel}

begin{document}
author[]{Βασίλης Μπισκετζής}  
title[]{Αυτόμοιοι Πιδακες σχετιζόμενοι με εκλάμψεις ακτίνων γ}
date{06/2019}

begin{frame}
maketitle
end{frame}

begin{frame}{Περιεχόμενα}
tableofcontents[hideallsubsections]
end{frame}

begin{frame}
section{Εισαγωγή}
1234
end{frame}

end{document}

As one can see from the above screenshot, the is still some space reserved for the title and author information. If you want the heading of the first section to appear without the vertical space befor it, you can redefine the beamer sidebar as follows:

documentclass[9pt]{beamer}
usetheme{Berkeley}
usepackage[utf8]{inputenc}
usepackage[english,greek]{babel}

makeatletter
  setbeamertemplate{sidebar beamer@sidebarside}
  {
    insertverticalnavigation{beamer@sidebarwidth}%
    vfill
    ifxbeamer@sidebarsidebeamer@lefttext%
    else%
      usebeamercolor{normal text}%
      llap{usebeamertemplate***{navigation symbols}hskip0.1cm}%
      vskip2pt%
    fi%
  }%
makeatother

begin{document}
author{Βασίλης Μπισκετζής}  
title{Αυτόμοιοι Πιδακες σχετιζόμενοι με εκλάμψεις ακτίνων γ}
date{06/2019}

begin{frame}
maketitle
end{frame}

begin{frame}{Περιεχόμενα}
tableofcontents[hideallsubsections]
end{frame}

begin{frame}
section{Εισαγωγή}
1234
end{frame}

end{document}