🐛 [RUM-11614][URL polyfill] Handle null iframe.contentWindow

[bcaudan]

Motivation

Telemetry error:

Cannot read properties of null (reading 'URL')
Cannot read properties of undefined (reading 'URL')

Changes

iframe.contentWindow can be null:

Let's fallback to globalObject in this case.

Test instructions

Let's see if the issue keeps appearing afterwards

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[datadog-prod-us1-3]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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