It's the miracle on Congress Street: Landlords are opening up unrented space to artists and allowing them to set up temporary galleries. The Arts District Partnership pioneered the idea 15 years ago, persuading landlords to let Phantom Galleries alight in empty storefronts. The scheme helped landlords by making their buildings look better, and helped artists by giving them exhibition space. Now the private sector is pitching in. Doug Biggers, former owner of the Tucson Weekly, is playing Kris Kringle this time around, allowing Chrissy Goral and the artists of Flash Gallery to squat legally-for a time-in the Rialto building storefronts. Their ephemeral gallery will last only until Biggers and co. renovate and sign up paying tenants, but optimists are hoping other landlords will follow suit.