Exporting batch jobs does not create a file with no entries anymore

[MDeLuise]

This PR fix the following issue: in the Web UI the "Export to CSV" feature present in the Batch Jobs section create a CSV file with empty entries.
In addition to this, it was optimised the close method of the JobTargetExporterCsv class.

Description of the solution adopted

• It's removed the osw.close() call present in the init method of the JobTargetExporterCsv class. That call closed the OutputStreamWriter before using it (causing the bug).
• The close of the OutputStreamWriter object is done when the JobTargetExporterCsv close is called.
• Since the writer.close() internally call writer.flush() the latter call is unnecessary, so it's removed from the close method of JobTargetExporterCsv.

[codecov]

Codecov Report

Merging #3310 (0583688) into develop (80954ba) will decrease coverage by 9.40%.
The diff coverage is n/a.

❗ Current head 0583688 differs from pull request most recent head 05f6d3c. Consider uploading reports for the commit 05f6d3c to get more accurate results

@@              Coverage Diff              @@
##             develop    #3310      +/-   ##
=============================================
- Coverage      55.30%   45.90%   -9.41%     
+ Complexity       868      776      -92     
=============================================
  Files           1516     1516              
  Lines          30463    30463              
  Branches        2528     2528              
=============================================
- Hits           16847    13983    -2864     
- Misses         12687    15725    +3038     
+ Partials         929      755     -174     

Impacted Files	Coverage Δ
...pse/kapua/kura/simulator/GatewayConfiguration.java	0.00% <0.00%> (-100.00%)	⬇️
...lipse/kapua/service/tag/internal/TagQueryImpl.java	0.00% <0.00%> (-100.00%)	⬇️
...a/kura/simulator/birth/BirthCertificateModule.java	0.00% <0.00%> (-100.00%)	⬇️
.../job/engine/exception/JobMissingStepException.java	0.00% <0.00%> (-100.00%)	⬇️
.../kapua/service/tag/internal/TagListResultImpl.java	0.00% <0.00%> (-100.00%)	⬇️
...ervice/account/internal/AccountListResultImpl.java	0.00% <0.00%> (-100.00%)	⬇️
...rvice/account/internal/KapuaAccountErrorCodes.java	0.00% <0.00%> (-100.00%)	⬇️
...rvice/endpoint/internal/EndpointInfoQueryImpl.java	0.00% <0.00%> (-100.00%)	⬇️
...scheduler/exception/SchedulerServiceException.java	0.00% <0.00%> (-100.00%)	⬇️
...cheduler/exception/SchedulerServiceErrorCodes.java	0.00% <0.00%> (-100.00%)	⬇️
... and 188 more