Allow empty needle in grapheme_str*() functions

[kocsismate]

Currently, string functions in ext/standard and ext/mbstring can accept an empty $needle, so we should add support for this in ext/intl as well.

The relevant code that needs to be changed is https://github.com/unicode-org/icu/blob/f744742e036441cbfd54b535e722dc3801ff0f63/icu4c/source/i18n/usearch.cpp#L2675. Discussion: php/php-src#5972 (comment)

[chschneider]

I made a patch to allow empty needles by explicitly checking in grapheme_strpos_utf16.

See
php/php-src@master...chschneider:grapheme_empty_needles
for the diff, should I submit a pull request or are there major issues with this approach?

[nikic]

@chschneider Looks reasonable