grapheme_substr inconsistency as compared to substr

[weierophinney]

grapheme_substr from Intl have differing behavior as compared to substr:

var_dump(substr('a', 1, null)); //  string(0) ""`
var_dump(grapheme_substr('a', 1, null)); // bool(false)

It looks like on php 5.x it also produces warning.

This behavior needs to be investigated against other wrappers and corrected if needed.

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Originally posted by @Xerkus at zendframework/zend-stdlib#73