fix: Don't allow empty values in allowed session types and scan all allowed scaling groups

[vesselofgod]

This is a follow-up to lablup/backend.ai#343
Related with: lablup/backend.ai-webui#1190

prevent allowed session types in empty and fix allowed session types check process handing

• Prevent allowed_session_types in empty status using trafaret.
  Change it not to be empty to solve problem of If the scaling group's allowed_session_types is empty, predicate check fail failure.
  
  

• Fix allowed_session_types check process handing
  Prevent stopping without checking all sgroups during the allowed_session_types check process

[codecov]

Codecov Report

Merging #542 (b85dcf5) into main (b947549) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #542      +/-   ##
==========================================
+ Coverage   48.86%   49.13%   +0.26%     
==========================================
  Files          54       54              
  Lines        9024     9031       +7     
==========================================
+ Hits         4410     4437      +27     
+ Misses       4614     4594      -20     

Impacted Files	Coverage Δ
src/ai/backend/manager/models/scaling_group.py	66.25% <ø> (ø)
src/ai/backend/manager/scheduler/predicates.py	52.52% <100.00%> (+25.35%)	⬆️

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