A hell of an incentive to build or sell your land to someone who will build. We've struggled with land speculation in the US for longer than we've been a country despite an obvious solution being available to fix this. We really could benefit from taxing land value alone, regardless of the value of any structures on top of it.
Incentives are a blunt instrument. They necessarily push in the wrong direction sometimes, because they’re dumb rules. Beware one-size-fits-all solutions.
Land that is zoned for some form of residential, commercial or industrial use but doesn't have building is considered to be vacant.
The US doesn't have a shortage of land. However, vacant properties (residential or commercial buildings with no tenants) are indeed a problem in densely populated regions.
For some areas, it's because the city population shrunk and nobody wants to live there (Detroit comes to mind as a semi-recent example) but for others it is speculative investors who don't want to be a landlord and are just betting the price will go up.
There's plenty of reasons to suggest that isn't the best purpose the land could serve, of course, but "vacant" wouldn't apply.