Allow empty configuration merging

[MatteoPologruto]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Code enhancement

What is the current behavior?

It is not possible to merge empty values to delete elements inside the configuration.

What is the new behavior?

It is possible to delete non-defaults elements inside the configuration using an empty value. A full json configuration must be passed to the merge command.

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 ✅

Comparison is base (95753fc) 68.85% compared to head (c7719c1) 68.86%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2432   +/-   ##
=======================================
  Coverage   68.85%   68.86%           
=======================================
  Files         204      204           
  Lines       20465    20470    +5     
=======================================
+ Hits        14091    14096    +5     
  Misses       5220     5220           
  Partials     1154     1154           

Flag	Coverage Δ
unit	68.86% <100.00%> (+<0.01%)	⬆️

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