Fix NPE for transaction id handling

[alex268]

Fixes #342

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 47.21%. Comparing base (fe3af55) to head (da4f249).

Files with missing lines	Patch %	Lines
...src/main/java/tech/ydb/query/impl/SessionImpl.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #344   +/-   ##
=========================================
  Coverage     47.21%   47.21%           
  Complexity     1750     1750           
=========================================
  Files           311      311           
  Lines         12499    12498    -1     
  Branches       1238     1237    -1     
=========================================
  Hits           5901     5901           
  Misses         6138     6138           
+ Partials        460      459    -1     

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