cmNinjaTargetGenerator: ensure that the object output directory exists

Ben Boeckelrequested to merge
ben.boeckel/cmake:ninja-always-make-object-dirs into master

If a target has no object compilations within it, nothing ensures that the directory exists yet it is still mentioned as an order-only dependency within the build files. While ninja is OK with this, samu, an alternate ninja implementation, is not. Make sure the directory always exists for consistency.

Also add a test case.

Fixes: #25526 (closed)

Maybe fine for %3.28.2?

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