fix: prevent NPE when isFinished() is called before DataDriver init

[shuwenwei]

Summary

Fix NullPointerException that occurs when isFinished() is called on DataDriver
before the query data source is properly initialized.

Problem

When DataDriver initializes but fails to acquire the read lock (returning
UNFINISHED_QUERY_DATA_SOURCE), calling isFinished() on the driver would still
invoke root.isFinished(). This triggers a chain of calls:
Driver.isFinished() -> SingleDeviceViewOperator.isFinished() -> hasNextWithTimer() -> ... -> AbstractSeriesScanOperator.hasNext() -> SeriesScanUtil.hasNextFile() -> dataSource.hasNextSeqResource() // NPE: dataSource is null

The root cause is that SeriesScanUtil.dataSource is only set during
initQueryDataSource(), which may not complete if the data source is unfinished.

Solution

Add an isInit() method to Driver and check it in isFinishedInternal():

• Driver.isFinishedInternal() now checks (isInit() && root.isFinished())
  instead of just root.isFinished()
• DataDriver.isInit() returns the init field
• SchemaDriver.isInit() returns true (always initialized)

This ensures that child operator methods are not called until the driver
is properly initialized.

Testing

Added DataDriverTest.testCallIsFinishedBeforeDataSourcePrepared() to verify
the fix works correctly when the query returns null data source.

[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 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 39.75%. Comparing base (e801885) to head (6e3be29).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
.../db/queryengine/execution/driver/SchemaDriver.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17440      +/-   ##
============================================
- Coverage     39.75%   39.75%   -0.01%     
  Complexity      312      312              
============================================
  Files          5135     5135              
  Lines        346996   347002       +6     
  Branches      44212    44213       +1     
============================================
- Hits         137959   137958       -1     
- Misses       209037   209044       +7     

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