Don't emit documentation for public import if no DDoc comment

[JinShil]

I believe I made a mistake in #9068

If a public import does not contain a DDoc comment, I still emitted it to the documentation file, just without the comment. I tested other symbols, and it appears that when a DDoc comment is absent, the symbol is not emitted to the documentation file. This PR makes the public imports work the same way.

I think this is the correct behavior, but if I'm mistaken, please let me know.

cc @thewilsonator @ZombineDev

[dlang-bot]

Thanks for your pull request and interest in making D better, @JinShil! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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[PetarKirov]

Can you add a test for that? Otherwise LGTM.

[JinShil]

Can you add a test for that?

Done. Thank you.