[ddc] Migrate ddb script to null safety
[ddc] Migrate ddb script to null safety

Issue: https://github.com/dart-lang/sdk/issues/46617
Change-Id: I34e9c11f6c721ca460a39f0037bf841afb6725f5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250352
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Commit-Queue: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk