fix(trace-viewer): Render loading dialog in middle of screen

[cpAdm]

• Simply omit position and margin whenever anchor is not provided to render dialog in middle of screen:

  

Closes: #38360

[github-actions]

Test results for "MCP"

1 failed
❌ [webkit] › mcp/test-debug.spec.ts:90 › test_debug (browser_snapshot/network/console) @mcp-ubuntu-latest

2609 passed, 116 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

11 flaky

⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1079 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node18`
⚠️ [firefox-page] › page/page-goto.spec.ts:83 › should work with Cross-Origin-Opener-Policy `@firefox-ubuntu-22.04-node18`
⚠️ [chromium-library] › library/inspector/cli-codegen-pick-locator.spec.ts:35 › should update locator highlight `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [webkit-library] › library/browsercontext-har.spec.ts:195 › should goBack to redirected navigation `@webkit-ubuntu-22.04-node18`
⚠️ [webkit-library] › library/browsercontext-har.spec.ts:208 › should goForward to redirected navigation `@webkit-ubuntu-22.04-node18`
⚠️ [webkit-library] › library/proxy.spec.ts:178 › should exclude patterns `@webkit-ubuntu-22.04-node18`
⚠️ [webkit-library] › library/selector-generator.spec.ts:574 › selector generator › should generate exact label when necessary `@webkit-ubuntu-22.04-node18`
⚠️ [webkit-library] › library/trace-viewer.spec.ts:205 › should show action context on locators and other common actions `@webkit-ubuntu-22.04-node18`
⚠️ [webkit-library] › library/video.spec.ts:378 › screencast › should capture navigation `@webkit-ubuntu-22.04-node18`
⚠️ [webkit-page] › page/page-autowaiting-basic.spec.ts:79 › should await form-post on click `@webkit-ubuntu-22.04-node18`
⚠️ [webkit-page] › page/page-history.spec.ts:55 › page.goBack should work for file urls `@webkit-ubuntu-22.04-node18`

40324 passed, 787 skipped

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Merge workflow run.