Conditionally setting a resource variable?

I've started using the mha-preview so I can include a fragment of the
message body in my mhonarc generated RSS feeds.

Until the database is properly populated with the preview data, I'm
getting a lot of output that includes $X-MSG-PREVIEW$.

Is there a way I can set X-MSG-PREVIEW to blank if it's not set?

Alternatively, is there a way I can tell mhonarc to not process a set of
lines if X-MSG-PREVIEW is not set?

Thanks!

david