[tw5] passing parameters to macro

[Bob Jansen]

How can I check if a parameter passed to a macro is blank?

I have tried

\define purchased(name_id)
<$list 
filter="[tag[Artworks]!tag[Index]!<currentTiddler>search:name_id{[<<__name_id__>>]is[blank]then[XX]}sort[title]]}>&bull;
 
<$link to={{!!artwork_id}}><$view field="title"/> <$view 
field="artwork_title"/></$link><br/></$list>
\end

but this does not work. If name_id is empty I want to search to look for 
the string XX which I know doesn't exist.

bobj

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