The documentation claims

    The increment/decrement operators do not affect
    boolean values. Decrementing NULL values has no
    effect too, but incrementing them results in 1. 
    http://php.net/manual/en/language.operators.increment.php

This might be considered inconsistent but changing this would be a rather random compatibility break.

Another script, further highlighting the inconsistence, and the fact it isn't an array bug, but one of unary decrease operator (or of the += and -= operators)

<?php
$a=array();
$b=array();
$c=null;
$d=null;

echo "\$a['z']--;\n";
$a['z']--;
var_export($a); print "\n\n";

echo "\$b['z']+=-1;\n";
$b['z']+=-1;
var_export($b); print "\n\n";

echo "\$c-=1;\n";
$c-=1;
var_export($c); print "\n\n";

echo "\$d--;\n";
var_export($d); print "\n\n";
?>

Result:
-------

$a['z']--;
array (
  'z' => NULL,
)

$b['z']+=-1;
array (
  'z' => -1,
)

$c-=1;
-1

$d--;
NULL