Fixed DSLParser error uncovered by ossfuzz

Missed one more case of Optional<Wrapper<Expression>>. This should be
the last one.

Bug: oss-fuzz:38944
Change-Id: Ic7f790cd99e2a3ee1c3874cc767a4702265d1723
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/450476
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
4 files changed