Move empty-checksum handling into case stmt

verify_checksum() {

  local checksum_command

  local filename="$1"

  local expected_checksum="$(echo "$2" | tr [A-Z] [a-z])"

  # If the specified filename doesn't exist, return success

  local filename="$1"

  [ -e "$filename" ] || return 0

  # If there's no expected checksum, return success

  local expected_checksum="$(echo "$2" | tr [A-Z] [a-z])"

  [ -n "$expected_checksum" ] || return 0

  case "${#expected_checksum}" in

  0) # If there's no expected checksum, return success

    return 0

    ;;

  32) # MD5

    [ -n "$HAS_MD5_SUPPORT" ] || return 0

    checksum_command="compute_md5"