Fixing issue related to issue #238

[rdingwell]

This fixes the issue with performing unions between lists when one of the lists is null as identified in #238 Pull requests into cql-execution require the following. Submitter and reviewer should ✔ when done. For items that are not-applicable, mark "N/A" and ✔.

CDS Connect and Bonnie are the main users of this repository. It is strongly recommended to include a person from each of those projects as a reviewer.

Submitter:

• [x] This pull request describes why these changes were made
• [x] Code diff has been done and been reviewed (it does not contain: additional white space, not applicable code changes, debug statements, etc.)
• [x] Tests are included and test edge cases
• [x] Tests have been run locally and pass
• [x] Code coverage has not gone down and all code touched or added is covered.
• [x] Code passes lint and prettier (hint: use yarn run test:plus to run tests, lint, and prettier)
• [n/a] All dependent libraries are appropriately updated or have a corresponding PR related to this change
• [x] cql4browsers.js built with yarn run build:browserify if source changed.

Reviewer:

Name:

• [x] Code is maintainable and reusable, reuses existing code and infrastructure where appropriate, and accomplishes the task’s purpose
• [x] The tests appropriately test the new code, including edge cases
• [ ] You have tried to break the code

[codecov-io]

Codecov Report

Merging #242 (8c2d1fe) into master (160b69e) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
- Coverage   92.37%   92.36%   -0.02%     
==========================================
  Files          47       47              
  Lines        4001     4006       +5     
==========================================
+ Hits         3696     3700       +4     
- Misses        305      306       +1     

Impacted Files	Coverage Δ
src/elm/overloaded.js	95.80% <100.00%> (-0.58%)	:arrow_down:

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