Fix graphme_substr for PHP >= 5.4.18 or >=5.5.1

Due the changes introduced in the intl extension for fix #62759 now the third argument returns an empty string if is null or 0

Open bug #66219 due the BC break introduced in the previous change.
Skip test if Intl extension is not loaded
Apply workaround just for null value
Merge pull request #5575 from Maks3w/hotfix/php-62759

Fix graphme_substr for PHP >= 5.4.18 or >=5.5.1
[#5575] Fix skip message

- s/intl/iconv/
Merge branch 'hotfix/5575'

Close #5575