Support changing null to false in update-bcd

[foolip]

trafficstars

Fixes https://github.com/openwebdocs/mdn-bcd-collector/issues/1042.

[foolip]

https://github.com/mdn/browser-compat-data/pull/23129 shows the changes that the collector can make after this fix.

[queengooborg]

This is looking good, but since the commits are signed, we can't merge this just yet! Could you sign the commits and re-push?

[foolip]

@queengooborg done!