How to fix Trigger function did not return a row in PostgreSQL

CREATE OR REPLACE FUNCTION my_trigger_function()
RETURNS TRIGGER AS $$
-- Function body here
$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION validate_user_insert()
RETURNS TRIGGER AS $$
BEGIN
  -- Validate or modify the NEW row
  IF NEW.email IS NULL THEN
    RAISE EXCEPTION 'Email cannot be null';
  END IF;
  
  -- Always return NEW for INSERT/UPDATE
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION audit_user_delete()
RETURNS TRIGGER AS $$
BEGIN
  -- Log the deletion
  INSERT INTO audit_log (action, user_id, old_data)
  VALUES ('DELETE', OLD.id, row_to_json(OLD));
  
  -- Always return OLD for DELETE
  RETURN OLD;
END;
$$ LANGUAGE plpgsql;

-- INCORRECT - missing RETURN in one branch
CREATE OR REPLACE FUNCTION bad_trigger()
RETURNS TRIGGER AS $$
BEGIN
  IF NEW.status = 'active' THEN
    RETURN NEW;
  ELSE
    -- Missing RETURN here!
  END IF;
END;
$$ LANGUAGE plpgsql;

-- CORRECT - all paths return
CREATE OR REPLACE FUNCTION good_trigger()
RETURNS TRIGGER AS $$
BEGIN
  IF NEW.status = 'active' THEN
    NEW.activated_at := NOW();
    RETURN NEW;
  ELSE
    RAISE EXCEPTION 'Status must be active';
  END IF;
END;
$$ LANGUAGE plpgsql;

-- AFTER trigger - return value is ignored
CREATE OR REPLACE FUNCTION notify_on_update()
RETURNS TRIGGER AS $$
BEGIN
  PERFORM pg_notify('user_updated', json_build_object('id', NEW.id)::text);
  RETURN NULL;  -- Return value ignored for AFTER triggers
END;
$$ LANGUAGE plpgsql;

-- Statement-level trigger - return NULL
CREATE OR REPLACE FUNCTION log_bulk_update()
RETURNS TRIGGER AS $$
BEGIN
  INSERT INTO audit_log (action) VALUES ('bulk_update');
  RETURN NULL;  -- Required for statement-level triggers
END;
$$ LANGUAGE plpgsql;