\documentclass[a4paper, 12pt]{article}

\begin{document}


\section{Paragraphs and line spacing}

Testing of spacing. The next music is surrounded by an empty line.
text text text text text text text text text text text text
text text text text text text text text text text text text

\begin{lilypond}
\score{ \relative c'{ c d e f g a b c} }
\end{lilypond}

text text text text text text text text text text text text
text text text text text text text text text text text text
text text text text text text text text text text text text

Next has no empty lines.
text text text text text text text text text text text text
text text text text text text text text text text text text
text text text text text text text text text text text text
\begin{lilypond}
\score{ \relative c'{ c d e f g a b c} }
\end{lilypond}
text text text text text text text text text text text text
text text text text text text text text text text text text

\end{document}