Make run return nonnullable FutureOr<int>

[robbecker-wf]

When migrating to null safety we missed making the run method return a non nullable FutureOr ... and that is causing weird casts and slight type mismatches in other tools. This fixes that since the exit code is either zero pass and non-zero for fail but should never be null.

[aviary2-wf]

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[robbecker-wf]

Closing in favor of a non-breaking change solution here https://github.com/Workiva/dart_dev/pull/407