fix read from iotdb data.

[ColinLeeo]

In IoTDB, the generated TsFile sets the encryptKey to null, but this case was not handled in the C++ implementation, resulting in a failure to read the data. This PR fixes the issue.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.

Project coverage is 65.79%. Comparing base (caa77ac) to head (a138b43).

Files with missing lines	Patch %	Lines
cpp/src/common/tsfile_common.h	60.00%	2 Missing ⚠️
cpp/src/common/tsfile_common.cc	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #520   +/-   ##
========================================
  Coverage    65.79%   65.79%           
========================================
  Files          566      566           
  Lines        33441    33446    +5     
  Branches      4691     4687    -4     
========================================
+ Hits         22002    22006    +4     
- Misses       10788    10789    +1     
  Partials       651      651           

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