core: list return empty array instead of null, when no cores are installed

[alessio-perugini]

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What kind of change does this PR introduce?

Changes the value of the json output when no core are installed

What is the current behavior?

Currently if no cores are installed we return a null value

What is the new behavior?

When no cores are installed we return an empty array []

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.67%. Comparing base (a0d912d) to head (82de269).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2691      +/-   ##
==========================================
- Coverage   67.77%   67.67%   -0.11%     
==========================================
  Files         234      234              
  Lines       22176    22176              
==========================================
- Hits        15030    15007      -23     
- Misses       5963     5990      +27     
+ Partials     1183     1179       -4     

Flag	Coverage Δ
unit	67.67% <100.00%> (-0.11%)	⬇️

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[per1234]

I verify this fixes #2690

Thanks Alessio!