don't attempt to sum NaN tonnage equipment

[NickAragua]

What it says in the title. Some spare parts in this particular warehouse have a "NaN" tonnage value ("0-shot inferno rounds"? lol), which was causing the total cargo tonnage to go to NaN, which prevented game loading.

Fix #5572

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.05%. Comparing base (d380134) to head (c691d88).
Report is 22 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5573      +/-   ##
============================================
- Coverage     10.05%   10.05%   -0.01%     
  Complexity     6070     6070              
============================================
  Files          1073     1073              
  Lines        141257   141258       +1     
  Branches      20600    20602       +2     
============================================
- Hits          14206    14203       -3     
+ Misses       125682   125680       -2     
- Partials       1369     1375       +6     

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