[OAS] Fix handling of schema.nullable to path and query params (#19…

…7046)

## Summary

Fixes handling of `schema.nullable(schema.object({..}))` to params and
query inputs. [Example in the
wild](https://github.com/jloleysens/kibana/blob/83e76cb4d854a3c3f9ffdaad8c6ee29d66d56710/x-pack/plugins/reporting/server/routes/common/generate/request_handler.ts#L33).


### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

packages/kbn-router-to-openapispec/src/oas_converter/kbn_config_schema/lib.test.ts

@@ -147,6 +147,30 @@ describe('convertPathParameters', () => {
       convertPathParameters(schema.object({ b: schema.string() }), { a: { optional: false } })
     ).toThrow(/Unknown parameter: b/);
   });
+
+  test('converting paths with nullables', () => {
+    expect(
+      convertPathParameters(schema.nullable(schema.object({ a: schema.string() })), {
+        a: { optional: true },
+      })
+    ).toEqual({
+      params: [
+        {
+          in: 'path',
+          name: 'a',
+          required: false,
+          schema: {
+            type: 'string',
+          },
+        },
+      ],
+      shared: {},
+    });
+  });
+
+  test('throws if properties cannot be exracted', () => {
+    expect(() => convertPathParameters(schema.string(), {})).toThrow(/expected to be an object/);
+  });
 });
 
 describe('convertQuery', () => {
@@ -166,10 +190,30 @@ describe('convertQuery', () => {
     });
   });
 
+  test('converting queries with nullables', () => {
+    expect(convertQuery(schema.nullable(schema.object({ a: schema.string() })))).toEqual({
+      query: [
+        {
+          in: 'query',
+          name: 'a',
+          required: false,
+          schema: {
+            type: 'string',
+          },
+        },
+      ],
+      shared: {},
+    });
+  });
+
   test('conversion with refs is disallowed', () => {
     const sharedSchema = schema.object({ a: schema.string() }, { meta: { id: 'myparams' } });
     expect(() => convertQuery(sharedSchema)).toThrow(/myparams.*not supported/);
   });
+
+  test('throws if properties cannot be exracted', () => {
+    expect(() => convertPathParameters(schema.string(), {})).toThrow(/expected to be an object/);
+  });
 });
 
 describe('is', () => {

packages/kbn-router-to-openapispec/src/oas_converter/kbn_config_schema/lib.ts

@@ -88,12 +88,17 @@ const convertObjectMembersToParameterObjects = (
   knownParameters: KnownParameters = {},
   isPathParameter = false
 ) => {
-  let properties: Exclude<OpenAPIV3.SchemaObject['properties'], undefined>;
+  let properties: OpenAPIV3.SchemaObject['properties'];
   const required = new Map<string, boolean>();
   if (isNullableObjectType(schema)) {
     const { result } = parse({ schema, ctx });
-    const anyOf = (result as OpenAPIV3.SchemaObject).anyOf as OpenAPIV3.SchemaObject[];
-    properties = anyOf.find((s) => s.type === 'object')!.properties!;
+    if (result.anyOf) {
+      properties = result.anyOf.find(
+        (s): s is OpenAPIV3.SchemaObject => !isReferenceObject(s) && s.type === 'object'
+      )?.properties;
+    } else if (result.type === 'object') {
+      properties = result.properties;
+    }
   } else if (isObjectType(schema)) {
     const { result } = parse({ schema, ctx });
     if ('$ref' in result)
@@ -108,6 +113,10 @@ const convertObjectMembersToParameterObjects = (
     throw createError(`Expected record, object or nullable object type, but got '${schema.type}'`);
   }
 
+  if (!properties) {
+    throw createError(`Could not extract properties from ${schema.describe()}`);
+  }
+
   return Object.entries(properties).map(([schemaKey, schemaObject]) => {
     const paramSchema = getParamSchema(knownParameters, schemaKey);
     if (!paramSchema && isPathParameter) {