Fix "Invalid argument supplied for foreach()" on a new PlaformRepository obj
Fix "Invalid argument supplied for foreach()" on a new PlaformRepository obj

This happens on "new PlatformRepository(array($somePackage))".
The parent constructor calls "\Composer\Repository\ArrayRepository::addPackage()",
which, on a brand new repo object, further calls "\Composer\Repository\PlatformRepository::initialize()"
and finally this iterates over a NULLd "$this->overrides", triggering the error.

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