Fix NULL vs. NULL comparison.

bin/pt-upgrade

@@ -10255,6 +10255,10 @@ sub identical_rows {
 
    my $n_vals = $size_array1 - 1;  # arrays are zero-indexed
    for my $i ( 0..$n_vals ) {
+      # NULL == NULL
+      # https://bugs.launchpad.net/percona-toolkit/+bug/1168434
+      next if !defined $array1->[$i] && !defined $array2->[$i];
+
       if ( defined $array1->[$i] && defined $array2->[$i] ) {
          return 0 unless $array1->[$i] eq $array2->[$i];
       }

t/pt-upgrade/samples/007/null_results/rows

@@ -1,33 +1,11 @@
 $rows = [
   [
     '1',
-    'a',
-    '2013-01-01 00:00:01'
+    'a'
   ],
   [
     '2',
-    'b',
-    '2013-01-01 00:00:02'
-  ],
-  [
-    '3',
-    'c',
-    '2013-01-01 00:00:03'
-  ],
-  [
-    '4',
-    'd',
-    '2013-01-01 00:00:04'
-  ],
-  [
-    '5',
-    'e',
-    '2013-01-01 00:00:05'
-  ],
-  [
-    '6',
-    'f',
-    '2013-01-01 00:00:06'
+    undef
   ]
 ];