Add allow_missing option to derive macro

[lirannl]

This can be helpful (but should not be enabled by default) for certain use cases.

Since it's off by default, it shouldn't compromise the safety of this macro.

[pyrossh]

Thanks @lirannl. A test case for this attribute would be good.

[lirannl]

I can write a single runtime unit test - but that's only whether it has 0 items when you try to use it on a non-existing folder.

Other than that, I'd need something like trybuild to do a compile-time test (make sure it fails to compile without the attribute, and succeeds with it)

[pyrossh]

LGTM. Thanks @lirannl!