AutoML.Net avoid empty dataset in trainValidationDatasetManager

[LittleLittleCloud]

We are excited to review your PR.

So we can do the best job, please check:

• There's a descriptive title that will make sense to other developers some time from now.
• There's associated issues. All PR's should have issue(s) associated - unless a trivial self-evident change such as fixing a typo. You can use the format Fixes #nnnn in your description to cause GitHub to automatically close the issue(s) when your PR is merged.
• Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• You have included any necessary tests in the same PR.
  Data classification&Value predication: Training failed when choosing "Split or File" validate strategy, "grid" tuner and enable "Subsample option". machinelearning-modelbuilder#2734

[codecov]

Codecov Report

Merging #6756 (b07828c) into main (8858ab6) will decrease coverage by 0.04%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6756      +/-   ##
==========================================
- Coverage   68.89%   68.85%   -0.04%     
==========================================
  Files        1216     1216              
  Lines      250915   250915              
  Branches    26259    26259              
==========================================
- Hits       172857   172762      -95     
- Misses      71238    71341     +103     
+ Partials     6820     6812       -8     

Flag	Coverage Δ
Debug	68.85% <ø> (-0.04%)	⬇️
production	63.34% <ø> (-0.05%)	⬇️
test	88.90% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

see 9 files with indirect coverage changes