There is no specific law requiring lighting, but property owners are required to take reasonable steps to make their property safe. Having outdoor lighting in an area frequented or used by people, like by the steps into/out of the building, is a reasonable step in most cases, unless the area was so well lit by other sources (e.g. lots of nearby street or parking lot lights) as to make a light by the door redundant. There is a good chance, therefore, that the building's owner (which may or may not be the business using the building--e.g. they may be renting) may be liable for not providing appropriate lighting. (Or sometimes the lessee/renter may be liable, if under the terms of their lease, they, not their landlord, are responsible for fitting out or maintaining outdoor lights.)