Handle null keys in gbek

[damccorm]

I noticed doing some broad validation that some transforms pass through a null key, and it triggers this condition, so this doesn't necessarily represent a decoding error like I initially thought. That should throw an exception anyways

Part of #36505

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[damccorm]

R: @claudevdm or @Abacn (I think you both have some context). It would be good to get this in before release cut since this breaks GBEK for some scenarios

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.91%. Comparing base (6dbbaa6) to head (d968b35).
⚠️ Report is 15 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #36505      +/-   ##
============================================
+ Coverage     55.08%   56.91%   +1.83%     
- Complexity     1666     3386    +1720     
============================================
  Files          1059     1221     +162     
  Lines        165352   186457   +21105     
  Branches       1190     3527    +2337     
============================================
+ Hits          91080   106128   +15048     
- Misses        72102    76972    +4870     
- Partials       2170     3357    +1187     

Flag	Coverage Δ
java	70.39% <ø> (+2.14%)	⬆️

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