Sandbox Visitors can fail on empty directories on non-Windows platforms

When a Sandbox Visitor checks a directory and it doesn't exist, it returns true on Windows and false on other platforms.

The code that uses the result of this check, IterateDirectory, doesn't continue processing if the result is false, leading to platforms like Linux not correctly populating the Sandbox.

Notably, the Windows version was changed to default true in https://github.com/EpicGames/UnrealEngine/commit/cac1fe0019a0ad973c2592852461f931868ef6e4.