Fix summary stats return logic

[BrianMichell]

Fixes a bug where if a processed chunk was fully padded traces (all 0's) the resulting SummaryStats values had NaN values instead of a faithful calculation of statistics.

[tasansal]

lgtm

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.77%. Comparing base (c3ba558) to head (acfe82c).
⚠️ Report is 195 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mdio/segy/_workers.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #716      +/-   ##
==========================================
+ Coverage   85.30%   88.77%   +3.46%     
==========================================
  Files          46       85      +39     
  Lines        2219     4607    +2388     
  Branches      306      300       -6     
==========================================
+ Hits         1893     4090    +2197     
- Misses        281      446     +165     
- Partials       45       71      +26     

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