Fix array_is_list call after unset

[VincentLanglet]

Extracted from #3381

It may be surprising but the code

$a = [1, 2, 3];
unset($a[2]);

var_dump(array_is_list($a));

display TRUE.

As shown in the example https://3v4l.org/0HFeL

So I updated ConstantArrayType::unsetOffsetType()
and ConstantArrayTypeBuilder::setOffsetValueType()

Edit: But I feel like this bugfix might not enough like shown by the latest added test 61b6160
=> See https://3v4l.org/XX2fU for the PHP behavior.

So maybe the fix should be done differently