Added NullType support

[alex268]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 45.85%. Comparing base (10c77c4) to head (e3cee70).

Files	Patch %	Lines
...src/main/java/tech/ydb/table/values/NullValue.java	40.00%	3 Missing ⚠️
...ch/ydb/table/result/impl/ProtoNullValueReader.java	66.66%	2 Missing ⚠️
.../src/main/java/tech/ydb/table/values/NullType.java	60.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #293      +/-   ##
============================================
+ Coverage     45.71%   45.85%   +0.14%     
- Complexity     1617     1638      +21     
============================================
  Files           300      303       +3     
  Lines         11846    11874      +28     
  Branches       1153     1153              
============================================
+ Hits           5415     5445      +30     
+ Misses         6017     6015       -2     
  Partials        414      414              

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