Ignore empty gzip timestamp in yearless log format helper

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.22%. Comparing base (e9af2a6) to head (0bf5f58).

Files	Patch %	Lines
plaso/lib/yearless_helper.py	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4829 +/- ## ========================================== - Coverage 85.22% 85.22% -0.01% ========================================== Files 426 426 Lines 38526 38532 +6 ========================================== + Hits 32835 32840 +5 - Misses 5691 5692 +1 ```

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