Null-assert the value given to a Completer expecting a non-null value

[srawlins]

The code with out the null assertion is legal as per the type signature, but will throw a runtime exception if the nullable value is null. To make this exception more explicit, the value must be null-checked before completing the completer with the value.

The analyzer will soon enforce such checks. See dart-lang/sdk#53253.

This PR is behaviorally a no-op.

Fixes flutter/flutter#136775

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.