fix(expect): if actual snapshot happens to not exist, don't crash

[agg23]

Fixes the crashing component of #36228.

page._expectScreenshot() has complicated logic to determine its return value and the returned type is not as typesafe as it should be. Under certain race conditions it is possible to timeout while still successfully taking a screenshot (similar to #36181).

Address the symptom and prevent Playwright from crashing when actual is nullish.

[github-actions]

Test results for "tests 1"

1 failed
❌ [webkit-library] › library/screenshot.spec.ts:205:14 › element screenshot › element screenshot should work with a mobile viewport @webkit-ubuntu-22.04-node18

6 flaky

⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:986:7 › cli codegen › should not throw csp directive violation errors @firefox-ubuntu-22.04-node18
⚠️ [playwright-test] › ui-mode-test-ct.spec.ts:59:5 › should run component tests after editing test @macos-latest-node18-1
⚠️ [webkit-library] › library/browsercontext-viewport-mobile.spec.ts:97:5 › mobile viewport › should fire orientationchange event @webkit-ubuntu-22.04-node18
⚠️ [webkit-library] › library/video.spec.ts:475:5 › screencast › should scale frames down to the requested size @webkit-ubuntu-22.04-node18
⚠️ [webkit-page] › page/page-screenshot.spec.ts:345:5 › page screenshot › should work while navigating @webkit-ubuntu-22.04-node18
⚠️ [playwright-test] › ui-mode-test-watch.spec.ts:145:5 › should watch all @windows-latest-node18-1

39351 passed, 820 skipped
✔️✔️✔️

Merge workflow run.

[deveshruttala-edu]

Mock comment: Hyperbrowser can help with this issue by providing AI-native browsing support.