minor fixes to sleeplog handling

[vincentvanhees]

Handle sleeplog with empty column names, fixes #1138 Specific to use of advanced sleeplog and recordings that start exactly at midnight, fixes #1188

Checklist before merging:

• [ ] Existing tests still work (check by running the test suite, e.g. from RStudio).
• [x] Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
• [x] Clean code has been attempted, e.g. intuitive object names and no code redundancy.
• [ ] Documentation updated:
  • [ ] Function documentation
  • [ ] Chapter vignettes for GitHub IO
  • [ ] Vignettes for CRAN
• [x] Corresponding issue tagged in PR message. If no issue exist, please create an issue and tag it.
• [x] Updated release notes in inst/NEWS.Rd with a user-readable summary. Please, include references to relevant issues or PR discussions.
• [ ] Added your name to the contributors lists in the DESCRIPTION file, if you think you made a significant contribution.
• [ ] GGIR parameters were added/removed. If yes, please also complete checklist below.

[vincentvanhees]

Closing this PR as it has issues that I have now addressed by rebuilding the fix in https://github.com/wadpac/GGIR/pull/1190