Fixed DSLParser unary expression error handling

Missed a case when eliminating optional/wrapper in an earlier CL.

Change-Id: If7f80ea6e2172acadf7b0087fe1a05853ccae445
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449838
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
4 files changed