[Hookup osd 2.0] AD Dashboards see 400 bad request issue

[ananzh]

Describe the bug
Hook up AD Dashboards with OSD 2.0 see the following error:

{"statusCode":400,"error":"Bad Request","message":"[request body]: expected value of type [any] but got [undefined]"}

To Reproduce
Run OpenSearch 2.0 with AD engine 2.0
Clone AD Dashboards current main (v1.3) to OpenSearch Dashboards plugins folder
Modify packages.json (delete the packages that have the compatibility issue):

"@testing-library/jest-dom": "^5.11.4",
"@testing-library/react": "^11.0.4"

Change opensearchDashboardsVersion to 2.0.0 in opensearch_dashboards.json
Run yarn osd bootstrap and yarn start in OpenSearch Dashboards

Expected behavior
No issue

Plugins
AD

[ananzh]

Issue is due to this line here

After typescript upgrade, Dashboards use a more restrict type check. For the get API from AD, we could see body is null.

Body null is transformed to undefined in the type check in the dashboards. Then, type undefined is no longer acceptable from type any by restricter type check.

There are two ways to change:

1. Seems request body is always null in get API. Could delete L64.
2. Change to body: schema.nullable(schema.any()),

Both would solve the issue.