[Fleet] fix schema validation to allow undefined/null (#202732)

Fix a few issues encountered with schema validation.

One of them reported here:
https://discuss.elastic.co/t/fleet-error-updating-policy-settings/371332

The other encountered locally when testing upgrades:
```
"Failed output validation: [request body.items.0.upgrade_details]: expected a plain object value, but found [null] instead."
```

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/fleet/common/types/models/agent_policy.ts

@@ -46,7 +46,7 @@ export interface NewAgentPolicy {
   global_data_tags?: GlobalDataTag[];
   monitoring_pprof_enabled?: boolean;
   monitoring_http?: {
-    enabled: boolean;
+    enabled?: boolean;
     host?: string;
     port?: number;
   };

x-pack/plugins/fleet/server/types/models/agent_policy.ts

@@ -105,7 +105,7 @@ export const AgentPolicyBaseSchema = {
   monitoring_pprof_enabled: schema.maybe(schema.boolean()),
   monitoring_http: schema.maybe(
     schema.object({
-      enabled: schema.boolean(),
+      enabled: schema.maybe(schema.boolean()),
       host: schema.maybe(schema.string({ defaultValue: 'localhost' })),
       port: schema.maybe(schema.number({ min: 0, max: 65353, defaultValue: 6791 })),
       buffer: schema.maybe(schema.object({ enabled: schema.boolean({ defaultValue: false }) })),