fix: Sorting validation and prevent NPE [DHIS2-16369]

[maikelarabori]

This PR aims to fix the recently added validation of the sorting attribute, in Visualization.
It should also allow dimension's uid present in columns -> items.
I've also made a small improvement to prevent NPEs.

[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8e9425d) 30.37% compared to head (dfe446a) 66.48%.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #16151       +/-   ##
=============================================
+ Coverage     30.37%   66.48%   +36.11%     
- Complexity    14227    31613    +17386     
=============================================
  Files          3508     3508               
  Lines        130769   130776        +7     
  Branches      15253    15254        +1     
=============================================
+ Hits          39721    86952    +47231     
+ Misses        87464    36754    -50710     
- Partials       3584     7070     +3486     

Flag	Coverage Δ
integration	50.27% <63.63%> (?)
integration-h2	32.59% <100.00%> (?)
unit	30.37% <9.09%> (-0.01%)	⬇️

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Files	Coverage Δ
...n/java/org/hisp/dhis/common/DimensionalObject.java	92.30% <100.00%> (+7.69%)	⬆️
...ava/org/hisp/dhis/visualization/Visualization.java	51.87% <100.00%> (+22.03%)	⬆️

... and 2180 files with indirect coverage changes

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