You could use:

/cast Purge
/cast Lightning Bolt

If there's nothing purgeable (sic) on the target, you'll get an error message on the screen (which can also be removed with a nifty little script) but it doesn't trigger the GCD so it'll drop through to Lightning Bolt.

You might find yourself getting stuck on pallies though - there used to be a bug (and probably still is) whereby Purge would cast if the pally has an aura, but not actually dispell it - that'll break this pretty quick and leave you scratching your head over your corpses

You could also replace /castsequence with /castrandom - some people prefer that.

I have purge and LB on different keys though, and that suits me fine.