fix: keep an empty-string key on unflatten instead of renaming it to 0

[spokodev]

getkey coerces a key to a number when it looks numeric, guarding only against NaN. But Number("") is 0 (not NaN), so a top-level empty-string key is turned into the index 0, which breaks the documented unflatten(flatten(obj)) round-trip:

unflatten(flatten({ "": 1, x: 2 }))   // { "0": 1, x: 2 }, expected { "": 1, x: 2 }
unflatten(flatten({ a: { "": 1 } }))  // { a: [1] }, expected { a: { "": 1 } }

flatten preserves "" losslessly (the key has no delimiter), so only unflatten corrupts it. The fix returns the empty string as-is before the numeric coercion:

function getkey (key) {
  if (key === "") return key
  const parsedKey = Number(key)
  ...
}

Numeric-index keys are unaffected (verified unflatten({"0":"a","1":"b"}) is unchanged). Full suite passes (51 + the new round-trip test); standard lint clean.