Fix NPE in table model export when -q is not specified

[CritasWang]

When using export-data with table model (sql_dialect=table) and only
specifying -db without -q, queryCommand is null. The original condition
sqlDialectTree && queryCommand == null only handled the tree model
case, causing the else branch to call queryCommand.trim().split(";")
which throws NullPointerException.

Restructure the branch logic to check queryCommand == null first:

• Tree model + no query: interactive SQL input (unchanged)
• Table model + no query: call exportBySql(null, 0) to auto-generate "select * from " for all tables, which also correctly applies start_time/end_time filters
• Query provided: split by semicolon for multi-statement support (unchanged)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.83%. Comparing base (1c1cbca) to head (d6b4fa5).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...in/java/org/apache/iotdb/tool/data/ExportData.java	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #17462   +/-   ##
=========================================
  Coverage     39.82%   39.83%           
  Complexity      312      312           
=========================================
  Files          5135     5135           
  Lines        347073   347075    +2     
  Branches      44220    44221    +1     
=========================================
+ Hits         138221   138252   +31     
+ Misses       208852   208823   -29     

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