Omit aggregatable_debug_reporting field in serialization if not
opted-in

The budget is required to be positive in source registration if
aggregatable debug reporting is opted-in. This change avoids outputting
a JSON that would fail to be validated by the header validator
(https://wicg.github.io/attribution-reporting-api/validate-headers).

Change-Id: I2cb6ca4f63c7066fd99ea64112dd756ffa9491ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5781782
Reviewed-by: Andrew Paseltiner <apaseltiner@chromium.org>
Commit-Queue: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1341051}