#480 pgfkeys behaves differently when using quotes

[[DEV] Solour]

commenting quotes produces and error that is not expected

\listfiles
\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{quotes}

\pgfkeys{
    /mytest/.is family, /mytest,
    arg/.store in = \myarg,
}

\newcommand{\mytest}[1]{%
    \pgfkeys{/mytest, #1}%  
    \textbf{\myarg}
}%

\overfullrule=5mm
\begin{document}


%% only fine with \usetikzlibrary{quotes} 
\mytest{
    arg={value}}

%% always fine
\mytest{%
    arg={value}}

\end{document}