26820 [canvaskit] Fix bug where empty scene doesn’t overwrite contentful scene (cla: yes, platform-web)

26822 Remove gn. ninja commands from test runner. (cla: yes)

26823 Remove built-in shadows from the @Config annotation (platform-android, cla: yes, waiting for tree to go green)

26825 Migrate //testing to nullsafety (cla: yes, waiting for tree to go green, tech-debt)

26826 Android lint to NNBD (cla: yes)

26827 Roll Skia from 3f0e25ca47ff to 7bf6bc0d0604 (5 revisions) (cla: yes, waiting for tree to go green)