✨ [REST-API] - restore possibility to update service/device configuration properties using a "default" value

[Agnul97]

Right now, if we try to update a service/device configuration property with a value equal to ““ we get an error from the back-end. In an earlier change I broke this behavior and now I want to restore it being able to set a property to the default value

In detail, the problem was caused by a known bug in the Collectors.toMap method https://stackoverflow.com/questions/24630963/nullpointerexception-in-collectors-tomap-with-null-entry-values

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 16.80%. Comparing base (2e475a1) to head (fa0d73d).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #4127   +/-   ##
==========================================
  Coverage      16.80%   16.80%           
  Complexity        22       22           
==========================================
  Files           2021     2021           
  Lines          52468    52468           
  Branches        4425     4425           
==========================================
+ Hits            8815     8819    +4     
+ Misses         43255    43251    -4     
  Partials         398      398           

Files with missing lines	Coverage Δ
...kapua/model/xml/adapters/XmlPropertiesAdapter.java	94.87% <100.00%> (ø)

... and 3 files with indirect coverage changes