SearchBuilder::flattenKeys throws Array offset Notice in PHP 7.4

Problem/Motivation

(#)
SearchBuilder::flattenKeys throws Array offset Notice in PHP 7.4

When flattenKeys method is invoked this code

$keys = array_filter($keys, function ($key) {
      return $key[0] !== '#';
    }, ARRAY_FILTER_USE_KEY);

generates the error shown.

Example:
Let's says $keys array looks like :

Array
(
    [0] => jstestsearch
    [#conjunction] => AND
)

key: 0 is considered as int and when we try to access $key[0] we are trying to access value considering it a string but it's recognised as an int hence the error.

Proposed resolution

(#)
Changing it to

$keys = array_filter($keys, function (string $key) {
      return $key[0] !== '#';
    }, ARRAY_FILTER_USE_KEY);

notice the argument hint:string in the function, this seems to work fine and fixes the error.