ext/Intl: Fix IntlIterator::current() on invalid iterators

[LamentXU123]

IntlIterator::current() could expose an undefined zval when called before theiterator is positioned, or after the iterator becomes invalid.

In these cases we directly returns the undefined zval and cause to a UNKNOWN:0 output in userland. Generally, a NULL output would be more proper.

<?php
$it = IntlBreakIterator::createWordInstance('en_US')->getPartsIterator();
var_dump($it->current());

output:

UNKNOWN:0

but I expected

NULL

Reproduce here

[devnexen]

that s quite a behavior change for a stable branch though

[LamentXU123]

Almost boarder line feature rather than a bug fix. Let's target master branch then.

[LamentXU123]

I by the way fix the doc CI by trimming the length into 100 though it is not related in this fix.