[v9] Disallow returning null from `beforeSendSpan` and also pass root spans

[lforst]

Description

In #14335 deprecated null to be returned from beforeSendSpan.

We should adjust the behaviour to either

• Noop and warn when null is returned
• Throw when null is returned

In either case the type needs to be updated.

Additionally, root spans also need to be passed to beforeSendSpan <-- This will require a migration/changelog entry.

[chargome]

Quick update: Since the param types of beforeSendSpan do not allow passing an event, we just discussed to try to see if this hook can be called earlier in the code (e.g. before creating the event). This would avoid adding all the code for destructuring the root span out of the event object and then writing back to it. see pr for details