[ACS-6804] Fix upload API file placement when relative path is undefined (#9342)

lib/js-api/src/api/content-custom-api/api/upload.api.ts

@@ -31,7 +31,7 @@ export class UploadApi extends NodesApi {
         const nodeBodyRequired = {
             name: fileDefinition.name,
             nodeType: 'cm:content',
-            relativePath
+            relativePath: relativePath ?? null
         };
 
         nodeBody = Object.assign(nodeBodyRequired, nodeBody);