[Tex/LaTex] Latex can not change page automatically

Question

If I only input the section title but no body text, Latex can not start new page automatically. The following is my minimal work.

\documentclass{book}

\begin{document}

\part{Title of Part I}
\chapter{Title of chapter a}
\section{section-1}
\section{section-2}
\section{section-3}
\section{section-4}
\section{section-5}
\section{section-6}
\section{section-7}
\section{section-8}
\section{section-9}
\section{section-10}

\section{section-11}
\section{section-12}
\section{section-13}
\section{section-14}
\section{section-15}
\section{section-16}
\section{section-17}
\section{section-18}
\section{section-19}
\section{section-20}

\end{document}

Answer 1

LaTeX prevents a page break right after \section to avoid a single section title at the bottom of the previous page.

Just add some dummy text until the section text is written, e.g.:

\part{Title of Part I}
\chapter{Title of chapter a}
\section{section-1}
TODO
\section{section-2}
TODO

...