Don't throw an error if a grouper encounters an empty leaf

[TahaAttari]

closes APHL-1121

[github-actions]

Formatting check succeeded!

[codecov]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes missing coverage. Please review.

Project coverage is 66.80%. Comparing base (a943655) to head (c0391b9).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...ava/org/opencds/cqf/fhir/utility/ExpandHelper.java	52.94%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #512      +/-   ##
============================================
- Coverage     66.80%   66.80%   -0.01%     
- Complexity     4042     4043       +1     
============================================
  Files           348      348              
  Lines         17890    17892       +2     
  Branches       2764     2764              
============================================
+ Hits          11951    11952       +1     
- Misses         4744     4745       +1     
  Partials       1195     1195              

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