Fix sign up broken because of missing workspace schema (#6013)

Allow workspace datasource factory to return null if the workspace
schema has not been created yet

packages/twenty-server/src/engine/twenty-orm/twenty-orm.manager.ts

@@ -35,6 +35,11 @@ export class TwentyORMManager {
       entities,
       workspaceId,
     );
+
+    if (!workspaceDataSource) {
+      throw new Error('Workspace data source not found');
+    }
+
     const entitySchema = this.entitySchemaFactory.create(entityClass);
 
     return workspaceDataSource.getRepository<T>(entitySchema);