CAS-14756: Fix AppleClang build errors in `SIImageStore::buildImage`
`std::min()` on macOS/AppleClang requires both arguments to be exactly
the same type; IPosition elements are ssize_t while the spatial cap
literal was int, causing deduction failure. Cast to Int before the
comparison.
TiledShape has no `isDefined()` method. Replace with a check on
`shape().nelements() > 0`, which is false for the default-constructed
(empty) TiledShape and true once initialized with an explicit shape.
