The gas station turns up in photos as late as 1940. Nighthawks is dated 1942. So perhaps the gas station was demolished and replaced with a diner in 1941. The city's taxmen photographed the corner again in 1980. In that photo, there is still no diner and no remnants of it, though the Esso station buildings were still standing there, graffitied and abandoned beneath a painted advertisement for London’s Hard Rock Cafe.
Link - Via Violins and Starships
Nobody is every likely to prove this conclusively since the artist never told.