How to check if a field exists in NEW in trigger

I have the following statement in a trigger:

    new.email = lower(new.email);

When I try to update a record without setting the email column however, I get an error:

SQL Error [42703]: ERROR: record "new" has no field "email"
  Where: SQL statement "SELECT lower(new.email)"
PL/pgSQL function on_record_modified() line 26 at assignment

I have seen some hacks suggesting TRY/CATCH or converting to a JSON and checking if the field exists, but I would think that there's a better way to check if the field is in the NEW record, no?

Any ideas?  Thanks!