Add missing return to nullably-typed functions

[srawlins]

Adds null statements at the end of functions which have implicit null returns. These functions all have nullable return types (and not void, dynamic`, and upcoming analysis encourages such functions to have explicit returns).

List which issues are fixed by this PR. You must list at least one issue. flutter/flutter#94759

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  writing and running engine tests.
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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

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[srawlins]

CC @yjbanov

[srawlins]

cc @mdebbar mind taking a look? Simple change, but I need two reviewers.

[dkwingsmt]

LGTM