Return predictions even in next_task even with empty model_version

[hlomzik]

Fixes two things:

• any empty version ("" or None) considered falsy
• if there are no model_version selected we return all predictions to be consistent with old version and GET /task/:id

Connected to LEAP-930 investigations

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 75.96%. Comparing base (e9f3d8d) to head (5f92622).
Report is 4 commits behind head on develop.

Files	Patch %	Lines
label_studio/tasks/models.py	50.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5685      +/-   ##
===========================================
+ Coverage    75.93%   75.96%   +0.02%     
===========================================
  Files          158      158              
  Lines        13324    13329       +5     
===========================================
+ Hits         10118    10125       +7     
+ Misses        3206     3204       -2     

Flag	Coverage Δ
pytests	75.96% <50.00%> (+0.02%)	⬆️

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