\makeatletter
\CheckCommand*\beamer@checkframetitle{\@ifnextchar\bgroup\beamer@inlineframetitle{}}
\renewcommand*\beamer@checkframetitle{\global\let\beamer@frametitle\relax\@ifnextchar\bgroup\beamer@inlineframetitle{}}
\makeatother

\addtobeamertemplate{frametitle}{\let\insertframetitle\insertsectionhead}{}
\addtobeamertemplate{frametitle}{\let\insertframesubtitle\insertsubsectionhead}{}