Handle the empty case in GrDrawingManager::insertTaskBeforeLast

Change-Id: I3c0f042b28cc39cc6df2f63b52a0f431cee6bda4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/421826
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed