Fixed possible NPE in OAuth2 Provider

[alex268]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 58.76%. Comparing base (f4a525f) to head (0da1abd).

Files with missing lines	Patch %	Lines
...ava/tech/ydb/auth/OAuth2TokenExchangeProvider.java	0.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #390      +/-   ##
============================================
- Coverage     58.78%   58.76%   -0.02%     
  Complexity     2170     2170              
============================================
  Files           320      320              
  Lines         12931    12935       +4     
  Branches       1267     1269       +2     
============================================
  Hits           7601     7601              
- Misses         4716     4718       +2     
- Partials        614      616       +2     

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