[external api] Set a default empty array for VpcFirewallRuleUpdate

[karencfv]

In our attempt to make the Go SDK generator less hacky oxidecomputer/oxide.go#283, we had the unexpected side effect of breaking our VpcFirewallRuleUpdateParams struct https://github.com/oxidecomputer/oxide.go/pull/283/files#diff-841a2b1b5f072b633551cac9c34c9732a722b415e297a1530b92d5988962062eR6810-R6811 due to Go's fun way of handling empty values. Using this API no longer allows us to delete the entirety of the firewall rules.

This commit sets an empty array as the default for rules following the pattern that other APIs use.
https://github.com/oxidecomputer/omicron/blob/main/nexus/types/src/external_api/params.rs#L1163-L1177

cc @sudomateo

[sudomateo]

I'm cool with this change to be consistent with other APIs but I would have assumed even without this change setting rules to [] in the request body would have worked. Did the previous configuration enforce a non-empty array?

Oh, I think I see it now. The Go SDK has omitempty on the rules attribute so setting [] serializes to null. The rules attribute isn't nullable in omicron so the omitempty will remain in the Go SDK. Now sending null in the request body will allow omicron to default rules to []. Do I have that right?