fix: lightgbm getting stuck when empty partition is chosen as the main worker in singleDatasetMode

[imatiach-msft]

Summary

Lightgbm can get stuck when SingleDatasetMode is enabled and if an empty partition is, by random chance, chosen as the main worker in the executor.
The fix is to only allow non-empty partitions to be chosen as the main worker.

Tests

Run the test "Verify LightGBM Classifier won't get stuck on empty partitions" in the file:
https://github.com/microsoft/SynapseML/blob/master/lightgbm/src/test/scala/com/microsoft/azure/synapse/ml/lightgbm/split1/VerifyLightGBMClassifier.scala#L610
many times, by random chance it will often get stuck after several runs.

Dependency chances

No dependency changes.

[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov-commenter]

Codecov Report

Merging #1458 (aca0398) into master (3eb4661) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1458      +/-   ##
==========================================
- Coverage   84.41%   84.31%   -0.10%     
==========================================
  Files         295      288       -7     
  Lines       14761    14633     -128     
  Branches      705      709       +4     
==========================================
- Hits        12460    12338     -122     
+ Misses       2301     2295       -6     

Impacted Files	Coverage Δ
...osoft/azure/synapse/ml/lightgbm/LightGBMBase.scala	95.78% <100.00%> (+0.03%)	⬆️
...rosoft/azure/synapse/ml/lightgbm/SharedState.scala	89.28% <100.00%> (+0.82%)	⬆️
...zure/synapse/ml/lightgbm/TaskTrainingMethods.scala	100.00% <100.00%> (ø)
...org/apache/spark/ml/param/JsonEncodableParam.scala	55.55% <0.00%> (-27.78%)	⬇️
...crosoft/azure/synapse/ml/io/http/HTTPClients.scala	67.64% <0.00%> (-8.83%)	⬇️
...g/apache/spark/ml/param/PythonWrappableParam.scala	66.66% <0.00%> (-8.34%)	⬇️
...re/src/main/python/synapse/ml/core/schema/Utils.py	67.10% <0.00%> (-5.27%)	⬇️
...soft/azure/synapse/ml/cognitive/TextToSpeech.scala	84.84% <0.00%> (-3.04%)	⬇️
...oft/azure/synapse/ml/cognitive/TextAnalytics.scala	86.20% <0.00%> (-2.69%)	⬇️
.../azure/synapse/ml/cognitive/TextAnalyticsSDK.scala	86.01% <0.00%> (-1.40%)	⬇️
... and 9 more

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[imatiach-msft]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).