Short answer: yes, in most cases you can leave an urban planter box empty over winter, and doing so is often safer than leaving it full of wet soil. An empty planter cannot hold a saturated block of compost that freezes and expands, which is one of the main reasons planters crack. However, empty does not mean risk-free. If rain, snow, or melting ice can collect inside the box and then freeze, the expanding ice can still split plastic, ceramic, concrete, wood, or composite walls.
The biggest factor is drainage. Before winter, remove old soil, roots, and debris, then check that every drainage hole is open. Do not plug the holes to keep water out; that can trap moisture inside. If possible, tilt the planter slightly, raise it off the ground with blocks, or move it to a sheltered spot so water runs away instead of pooling in the bottom. A breathable cover over the top can keep snow and rain out while still allowing some airflow, but avoid sealing the planter tightly with plastic, because trapped condensation can create the same freeze-thaw problem.
Material matters too. Plastic and resin planters are usually the most forgiving, though they can become brittle in extreme cold and should not be knocked or dragged. Wooden planters may dry and shrink when left empty, which can open joints or cause surface cracks, so clean and inspect them before winter and avoid sealing them while damp. Ceramic, terracotta, and concrete planters are porous and much more vulnerable to freeze-thaw damage; an empty, well-drained box is safer, but if the planter is expensive or already has hairline cracks, move it into a garage, shed, or other frost-free shelter. Metal planters will not usually crack, but standing water can cause rust, so empty and dry them well.
An empty planter will not crack simply because it is empty. It cracks when water gets trapped in the material, in the bottom, or in old soil and then freezes. For most urban balconies, rooftops, and patios, the practical routine is simple: empty the box, clear the drainage holes, lift or tilt it, cover the top loosely if rain and snow collect, and secure lightweight planters against winter wind. If the planter is fragile, very large, or already damaged, store it under cover or choose a smaller, frost-safe container for winter. So you can leave it empty, but do not leave it full of soggy soil, blocked drainage, or standing water.