Ada seems to set TYPE_SIZE of an ARRAY_TYPE to integer_zero_node, Not integer_zero_node, but to bitsize_zero_node, for any type that has a size of zero, which is the case here
domain <integer_type 0x401dcf18> min <integer_cst 0x401c25e8 6> max <integer_cst 0x401c25d0 5> since the size in elements of the ARRAY_TYPE is zero. Note that ACATS tests are not testing *typical* Ada usage, but usage at the fringes of the language, so things like zero-sized types or array with bounds of 2**31 - 10 to 2**31 - 1 are quite common!