Fix 1588: NPE printing or previewing DropShip

[Sleet01]

Adds a safety check for the MML print code to prevent an NPE when an Aerospace unit's Engine isn't populated.
This shouldn't happen, and is nominally controlled by MM code, but if it does, there's no real reason not to populate the sheet:

• DropShips don't even show the Engine info line,
• ASFs do, but it's informational, not functional

Testing:

• Ran checks with provided DS .blk files
• Ran all 3 projects' unit tests
• Added a unit test to confirm proper functionality in the above case.

Close #1588

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 2.14%. Comparing base (91bb560) to head (f81edf9).
Report is 134 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #1601      +/-   ##
===========================================
+ Coverage      0.65%   2.14%   +1.48%     
- Complexity       64     203     +139     
===========================================
  Files           266     266              
  Lines         30545   30547       +2     
  Branches       5206    5207       +1     
===========================================
+ Hits            201     656     +455     
+ Misses        30304   29737     -567     
- Partials         40     154     +114     

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[repligator]

Did a bit of testing this morning with the latest MM/MML. You can reliably reproduce the NPE from #1588 by going to new Warship, changing the Vessel Type to Subcompact Warship, going to preview, and pressing Record Sheet.

When I tested under this PR, the NPE is no longer thrown when repeating the above steps.