Enable strictNullChecks for integration tests

[tomduncalf]

What, How & Why?

This change is required for flexible sync. Ideally we'd enable strict: true but this throws a lot more errors so just doing strictNullChecks for now.

I've tried to do runtime assertions where I thought it might make sense (e.g. there's a chance the thing might not exist, if something is badly broken, so throw) and otherwise use non-null assertions, but happy to change if people think we should take a different approach.

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 🚦 Tests
• 📱 Check the React Native/other sample apps work if necessary
• 📝 Public documentation PR created or is not necessary
• 💥 Breaking label has been applied or is not necessary

If this PR adds or changes public API's:

• typescript definitions file is updated
• jsdoc files updated
• Chrome debug API is updated if API is available on React Native

[takameyer]

I think the test should make assertions instead of throwing exceptions. Maybe this warrants a quick discussion.

[kraenhansen]

I would love for me or you (probably me) to do a follow-up PR making this strict instead of simply strictNullChecks.

[tomduncalf]

If you have the time, that would be awesome, but if not I am happy to look at it once my current bit of work is out of the way (probably end of the week). I'll create an issue to track

[tomduncalf]

#4200