Since {WasmGraphBuildingInterface::UnOp} always sets the result value
node, we have to ensure the passed {result} is not null.
Note: This can never happen with the current set of operators; this is
guarding against possible future operators.

Bug: chromium:1285041
Change-Id: I657cb02a9c650121a51ee795ef5572a5abd66933
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3447373Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78995}