How can I test for the existence of something in jexl?

I've set lenient mode off in order to get more information - essentially
lenient mode suppresses any interesting errors.  But without lenient mode
there seems to be no way to test the presence or absence of a variable.
 I'm looking for something like:

if (foo?) { /* ok foo is a variable, might be null, but that's ok */ }

Is this possible?  It is common in template languages, I'm surprised it
isn't here.

Jeff

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