The output doesn't contain a null character; its just an artifact of the way od displays contents in units of 2 bytes:

$ echo "ab" > /tmp/foo
$ ls -l /tmp/foo
-rw-r--r-- 1 davem davem 3 Dec 18 16:16 /tmp/foo
$ od -xc /tmp/foo
0000000 6261 000a
          a   b  \n  \0
0000003
$ echo "abc" > /tmp/foo
$ ls -l /tmp/foo
-rw-r--r-- 1 davem davem 4 Dec 18 16:17 /tmp/foo
$ od -xc /tmp/foo
0000000 6261 0a63
          a   b   c  \n
0000004
$
[download]

(Note that the first one is length three, but od displays a fourth null byte.)

Dave.