Allow OverlayStream overlay at offset 0

[yunzheng]

Previously, OverlayStream would ignore overlays added at offset 0 because of an incorrect truthiness check (if next_overlay_offset:). This caused the offset 0 to be treated as False (no overlay).

This PR changes the check to explicitly verify if next_overlay_offset is not None:.
It also adds a regression test to ensure overlays at offset 0 work as expected.

fixes #115

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (ab87cdf) to head (9fe2698).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/util/stream.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #116   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         27      27           
  Lines       1967    1967           
=====================================
  Misses      1967    1967           

Flag	Coverage Δ
unittests	0.00% <0.00%> (ø)

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[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing bugfix/overlay-offset-0 (9fe2698) with main (ab87cdf)}

Summary

✅ 28 untouched benchmarks