[11.x] Add tests to improve test coverage for Arr::whereNotNull

[saMahmoudzadeh]

This PR, adds tests to improve the test coverage of Arr::whereNotNull.

The array contains only numeric items

$array = array_values(Arr::whereNotNull([1, 2, 3]));
$this->assertEquals([1, 2, 3], $array);

The array contains only null values

$array = array_values(Arr::whereNotNull([null, null, null]));
$this->assertEquals([], $array);

An array consisting of a mix of characters and null values.

$array = array_values(Arr::whereNotNull(['a', null, 'b', null, 'c']));
$this->assertEquals(['a', 'b', 'c'], $array);

An array containing various data types.

$array = array_values(Arr::whereNotNull([null, 1, 'string', 0.0, false, [], new stdClass(), function () {}]));
$this->assertEquals([1, 'string', 0.0, false, [], new stdClass(), function () {}], $array);