Handling mipmapaccess failing

This was checked for in the legacy shader (in a diff way), but not in
the new backends. This null-pattern seemed the clearest.

Change-Id: Icbf9424e8c9444466e1cea5139944504df352c63
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/341416
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed