Return null rather than fall off nullable onError catchError handler.

[srawlins]

Ignore body_might_complete_normally_catch_error violations

Related to #105750

This onError catchError should return a value.

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[christopherfujino]

LGTM

[AlexV525]

@srawlins Could you do the same in previous TODOs? Some of them might require for more reasonable fix which affects behavior but some of them are not.

[AlexV525]

LGTM