Incompatibility of inmargin theme with `usepdftitle=false` option (fix #885)

[samcarter]

Provisional fix - in a very specific situation, the author will be missing on the title page:

\documentclass[
usepdftitle=false
]{beamer}

\author[]{Lecturer Name}

\usetheme{Bergen}

\begin{document}
\maketitle
\end{document}