Fix Service Crashing when Returning Nullable, Union Type Arrays

[DimuthuMadushan]

Purpose

Fixes: #2571
Fixes: #2578

Examples

N/A

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #565 (78ee73a) into master (0d463c7) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #565      +/-   ##
============================================
+ Coverage     93.84%   93.88%   +0.03%     
- Complexity      346      352       +6     
============================================
  Files            53       53              
  Lines          3689     3712      +23     
  Branches       1962     1968       +6     
============================================
+ Hits           3462     3485      +23     
  Misses          199      199              
  Partials         28       28              

Impacted Files	Coverage Δ
ballerina/response_formatter.bal	97.29% <100.00%> (+0.93%)	⬆️
...ina/stdlib/graphql/runtime/engine/EngineUtils.java	98.55% <100.00%> (+0.08%)	⬆️
...dlib/graphql/runtime/engine/ResponseGenerator.java	95.80% <100.00%> (+0.56%)	⬆️
ballerina/engine.bal	96.96% <0.00%> (-1.50%)	⬇️

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