fix: validation errors when no scopes configured

[MichaelGoberling]

Users with no scopes configured were getting validation errors:

Error: [generateOAuthSTSAuthToken] Validation errors: [
  {
    ...,
    "message": "Invalid scopes format. Must be a comma separated list of scopes (e.g. scope1, scope2, scope3 or scope1,scope2,scope3)"
  }
]

This case should actually result in the action using the default scopes (from I/O Management API), however core.getInput() returns an empty string for un-configured inputs, which was failing validation. We can just make scopes undefined so it gets ignored during validation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (357281a) to head (a2dbbc5).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #66   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          107       107           
  Branches        20        21    +1     
=========================================
  Hits           107       107           

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