Answer by Schrödinger's cat for Tikz node without text

There is the key node contents that specifically allows you to drop the empty braces, {}. A minor pitfall is that this key needs to come after the name of the node.

\documentclass[tikz,border=3mm]{standalone}
\begin{document}
\begin{tikzpicture}[circle only/.style={circle,draw,inner sep=2pt,node contents={}}]
 \path node (P) [circle only] ;
 %\path node[circle only] (P)  ; % does not work
 \draw (P.north west) -- ++ (-1,1);
\end{tikzpicture}
\end{document}

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For somewhat more advanced options look up the definition of vertex that ships with tikz-feynman.