Allow to create empty OmniArray
Summary:
This fixes the following issue:
ArgumentCountError : Too few arguments to function Keruald\OmniTools\Collections\OmniArray::__construct(), 0 passed in C:\dev\nasqueron\keruald\omnitools\src\Strings\Multibyte\OmniString.php on line 98 and exactly 1 expected
Test Plan: New test for OmniString::explode, which does trigger this code path.
Reviewers: dereckson
Differential Revision: https://devcentral.nasqueron.org/D2491