Don't fill last render task map with nullptrs

Bug: skia:10320
Change-Id: I8d388c273f37f3dd104a2940032966b477d9a92a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295199
Commit-Queue: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed