Add a default version_comparable value to fix error thrown in APIs migrated from the version which doesn't support API Revisions

[RusJaI]

Fixes : wso2/api-manager#1886

[codecov]

Codecov Report

Patch coverage: 25.00% and project coverage change: +2.89 🎉

Comparison is base (3faade0) 45.22% compared to head (6a34493) 48.12%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12036      +/-   ##
==========================================
+ Coverage   45.22%   48.12%   +2.89%     
==========================================
  Files        1781     1781              
  Lines      127921   127963      +42     
  Branches    18304    18314      +10     
==========================================
+ Hits        57851    61578    +3727     
+ Misses      63255    59230    -4025     
- Partials     6815     7155     +340     

Flag	Coverage Δ
integration_tests	42.77% <25.00%> (+3.35%)	⬆️
unit_tests	20.92% <0.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
...on/apimgt/persistence/RegistryPersistenceImpl.java	59.59% <25.00%> (+0.22%)	⬆️

... and 273 files with indirect coverage changes

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