No errors with express-validator isEmpty
check('firstName', 'First Name is required').isEmpty() is enforcing firstName to be empty.
You will want to change that last part to .not().isEmpty(), so that you invert the isEmpty validator.
Might also be of your interest: https://github.com/express-validator/express-validator/issues/476
In your snippet you have not added a validtor to check if a value is not empty.You could achieve this by doing:
check('firstName', 'First Name is required').notEmpty().notEmpty() adds a validator to check if a value is not empty; that is, a string with a length of 1 or bigger.
https://express-validator.github.io/docs/validation-chain-api.html#notempty