Description:
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null assignment to a variable and refer to it as an array element does not return any errors

after assign null to variable var_dump return null and isset return false, but why in line 5 assign first element of $zonk variable(which is don't array and don't should exists at all) to next variable doesnt't return any errors

Test script:
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$zonk = null;            //1
var_dump($zonk);         //2 return null
var_dump(isset($zonk));  //3 return false

$beret = $zonk[0];       //5 don't throw nothing ???
var_dump($beret);        //6 return null
var_dump(isset($beret)); //7 return false

Expected result:
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is line 6 don't should throw something to output error or notice?
or if it is good behavior then in line 3 isset should return true?
or if i'm wrong explain why? becouse i can't figured it out