guard against nullptr while drawing
guard against nullptr while drawing

I was unable to reproduce the fuzz, but
DirectMaskSubRunNoCache::Make can certainly return nullptr.
Draw nothing when a nullptr is passed.

Bug: oss-fuzz:33618

Change-Id: I6aaa954b87c7d2b19fa96ff5d66de4083a589580
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/401516
Commit-Queue: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Reviewed-by: Robert Phillips <95e8ac5504948c7bf91b22c16a8dbb7ae7c66bfd@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia