Fix 5796: Safety check for entities with no hex location, e.g. just-loaded infantry

[Sleet01]

Prevents the NPE when hex is null due to the unit having just been loaded the previous Movement phase.

Testing:

• Ran saves from OP with and without fix, and confirmed via debugging that the just-loaded unit was at fault.
• Ran all 3 projects' unit tests

Close #5796

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.05%. Comparing base (8b06aec) to head (f797f83).
Report is 34 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5814      +/-   ##
============================================
- Coverage     29.06%   29.05%   -0.01%     
+ Complexity    13913    13911       -2     
============================================
  Files          2507     2507              
  Lines        266339   266366      +27     
  Branches      47611    47621      +10     
============================================
- Hits          77403    77400       -3     
- Misses       185012   185033      +21     
- Partials       3924     3933       +9     

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