add the return code of the evaluated expression

[roc007]

zshdb<0> l
  1:    #!/bin/zsh -f
  2:
  3: => if test -z "$a" ; then
  4:        echo "a is not set"
  5:    else
  6:        echo "a is set to $a"
  7:    fi
zshdb<0> ev?
eval: test -z "$a"
$? is 0
zshdb<0> ev a=22
$? is 0
zshdb<0> ev?
eval: test -z "$a"
$? is 1
zshdb<0>