If a landlord has a storefront that he's letting sit empty, paying the mortgage on the property (and also the existing taxes, if they aren't part of the mortgage), and the insurance, and the security... why do you think that a land tax is going to change things? Especially if a land tax replaces a land-plus-improvements tax, which may come out a wash for many storefront-owners? (And, if it's a ground-level storefront in a highrise, their taxes may go down, because the land is a lower part of the value of their property.)
Don't change the incentives (very much) for this particular behavior, and watch the behavior not change.
Don't change the incentives (very much) for this particular behavior, and watch the behavior not change.