[external api] Set a default empty array for VpcFirewallRuleUpdate (#8175)

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

Author: karencfv

common/src/api/external/mod.rs

@@ -1789,6 +1789,7 @@ pub struct VpcFirewallRuleUpdate {
 #[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
 pub struct VpcFirewallRuleUpdateParams {
     #[schemars(length(max = 1024))]
+    #[serde(default)]
     pub rules: Vec<VpcFirewallRuleUpdate>,
 }
 

openapi/nexus.json

@@ -25265,16 +25265,14 @@
         "type": "object",
         "properties": {
           "rules": {
+            "default": [],
             "type": "array",
             "items": {
               "$ref": "#/components/schemas/VpcFirewallRuleUpdate"
             },
             "maxItems": 1024
           }
-        },
-        "required": [
-          "rules"
-        ]
+        }
       },
       "VpcFirewallRules": {
         "description": "Collection of a Vpc's firewall rules",