^@ is how your editor represents the NUL character.
In the file, each of those ^@ is the single byte chr(0).

>perl -e"printf qq{^%s %2d\n}, chr($_+64), $_ for 0..31"
^@  0
^A  1
^B  2
..
^Y 25
^Z 26
^[ 27
^\ 28
^] 29
^^ 30
^_ 31
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