[texhax] empty arguments

[Adam Fenn]

I have a simple Plain TeX macro which allows me to input place-names with 
their grid references
in the form \pn{place-name}{1.23456 2.23456} but typesets place-name 123456 
223456

\def\pn#1#2{\noindent#1\hfill\pnAux#2\relax}%
\def\pnAux#1.#2 #3.#4\relax{#1#2\quad#3#4}%


However  \pn{place-name}{} produces the complaint


! Paragraph ended before \pnAux was complete.


How can I modify the macro so that it can accept an empty second argument?


Adam Fenn