[Helper] FileSystem::createDirectory: fix missing return warning

[fredroy]

I guess it was not implemented because it should never happen (the error would be detected at the stat() stage)

My beef is more about the fact that createDirectory() return true if there is an error 🤔
which is counter-intuitive ; I would expect return false in case of an error... 🫠

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[bakpaul]

Linked to this : #5924