test_DButils: clarify no-output tests

[jtniehof]

Tiny PR relevant to discussion in #94, to clarifly the zero output product ID in a test is meant to verify the fix didn't introduce a regression, it's not actually a no-output process

PR Checklist

• Pull request has descriptive title
• Pull request gives overview of changes
• New code has inline comments where necessary
• (N/A) Any new modules, functions or classes have docstrings consistent with dbprocessing style
• (N/A) Major new functionality has appropriate Sphinx documentation
• (N/A) Added an entry to release notes if fixing a major bug or providing a major new feature
• (N/A) New features and bug fixes should have unit tests
• Relevant issues are linked in the description (use Closes # if this PR closes the issue, or some other reference, such as See # if it is related in some other way)