prefer-at: remove unsafe autofix for .slice with 1 argument

[k-yle]

It's not safe to change array.slice(-n)[0] to array.at(-n). This code is not equivalent for arrays with only 1 item. It's only equivalent for .slice(-1)

Proof:

testCases = [ 'a', 'ab', 'abc', 'abcd' ];
testCases.map(array => ({
    original: array.slice(-2)[0],
    autofixed: array.at(-2),
}));

// output:
[ 
  {original: 'a', autofixed: undefined}, // 🔴 this is different
  {original: 'a', autofixed: 'a'},
  {original: 'b', autofixed: 'b'},
  {original: 'c', autofixed: 'c'},
]

[fisker]

This is incorrect, the value of startIndexNode doesn't matter. It only result differently when the index is out of the boundary of string. And sting don't have .shift/.pop, so it doesn't matter.

[k-yle]

@fisker we're checking startIndexNode because .slice(-1)[0] can still be safely converted to .at(-1), but only for -1, not any other number. For any other number the behaviour is different if the index is out of bounds

[fisker]

You are right.

But i think we should still report array.slice(-9)[0], just remove the autofix.