[Saved Objects] Fix document migrator to prevent losing namespaces on import

packages/core/saved-objects/core-saved-objects-migration-server-internal/src/document_migrator/document_migrator.test.ts

@@ -713,6 +713,36 @@ describe('DocumentMigrator', () => {
         ]);
       });
 
+      it('does not lose namespaces in documents with undefined namespace and defined namespaces property', () => {
+        const migrator = new DocumentMigrator({
+          ...testOpts(),
+          typeRegistry: createRegistry(
+            { name: 'dog', namespaceType: 'multiple', convertToMultiNamespaceTypeVersion: '1.0.0' }
+            // no migration transforms are defined, the typeMigrationVersion will be derived from 'convertToMultiNamespaceTypeVersion'
+          ),
+        });
+        migrator.prepareMigrations();
+        const obj = {
+          id: 'mischievous',
+          type: 'dog',
+          attributes: { name: 'Ann' },
+          coreMigrationVersion: kibanaVersion,
+          typeMigrationVersion: '0.1.0',
+          namespaces: ['something'],
+        } as SavedObjectUnsanitizedDoc;
+        const actual = migrator.migrateAndConvert(obj);
+        expect(actual).toEqual([
+          {
+            id: 'mischievous',
+            type: 'dog',
+            attributes: { name: 'Ann' },
+            coreMigrationVersion: kibanaVersion,
+            typeMigrationVersion: '1.0.0',
+            namespaces: ['something'],
+          },
+        ]);
+      });
+
       it('does not fail when encountering documents with coreMigrationVersion higher than the latest known', () => {
         const migrator = new DocumentMigrator({
           ...testOpts(),

packages/core/saved-objects/core-saved-objects-migration-server-internal/src/document_migrator/internal_transforms.ts

@@ -105,6 +105,13 @@ function convertNamespaceType(doc: SavedObjectUnsanitizedDoc) {
   const { namespace, ...otherAttrs } = doc;
   const additionalDocs: SavedObjectUnsanitizedDoc[] = [];
 
+  if (namespace == null && otherAttrs.namespaces) {
+    return {
+      additionalDocs,
+      transformedDoc: otherAttrs,
+    };
+  }
+
   // If this object exists in the default namespace, return it with the appropriate `namespaces` field without changing its ID.
   if (namespace === undefined) {
     return {