Fix NPE when encoding continous nulls in Tablets

[jt2594838]

To increase encoding efficiency, when encoding nulls in a Tablet, the previous value will be used as the placeholder for nulls.
When the previous value itself is null, an exception will be raised.

Instead of using the previous value, the last non-null value (or a default value if none exists) is used as the placeholder.

[sonarqubecloud]

Quality Gate failed

Failed conditions
9.2% Duplication on New Code (required ≤ 5%)

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 27.27273% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.95%. Comparing base (7384701) to head (893cdd0).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/org/apache/iotdb/session/util/SessionUtils.java	27.27%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #16107   +/-   ##
=========================================
  Coverage     38.94%   38.95%           
  Complexity      198      198           
=========================================
  Files          4878     4876    -2     
  Lines        318186   318187    +1     
  Branches      40126    40128    +2     
=========================================
+ Hits         123923   123939   +16     
+ Misses       194263   194248   -15     

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