Fix LightningDataModule zero-length attribute

[littlebullGit]

What does this PR do?

• Adds a class-level fallback allow_zero_length_dataloader_with_multiple_devices so custom [LightningDataModule] subclasses remain compatible even if they skip super().__init__().
• Adds regression coverage ensuring Trainer loops can validate with such datamodules.
• Updates the changelog with the fix summary.

Fixes #21358

Before submitting

• Was this discussed/agreed via a GitHub issue? ✅ (Missing property 'allow_zero_length_dataloader_with_multiple_devices' of LightningDataModule #21358)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (not needed)
• Did you write any new necessary tests? (new regression test)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request? (none)
• Did you update the CHANGELOG? (added “Fixed” entry)

---

📚 Documentation preview 📚: https://pytorch-lightning--21383.org.readthedocs.build/en/21383/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82%. Comparing base (b09e96e) to head (300c41e).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (b09e96e) and HEAD (300c41e). Click for more details.

HEAD has 642 uploads less than BASE

Flag	BASE (b09e96e)	HEAD (300c41e)
cpu	176	30
lightning_fabric	44	0
pytest	88	0
python3.12	53	9
python3.12.7	52	9
lightning	87	15
python3.11	36	6
python3.10	17	3
python	18	3
pytorch_lightning	45	15
pytorch2.7	9	3
pytest-full	88	30
pytorch2.2.2	9	3
pytorch2.6	9	3
pytorch2.4.1	8	3
pytorch2.5.1	8	3
pytorch2.8	9	3
pytorch2.1	18	6
pytorch2.3	9	3
pytorch2.9	9	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21383     +/-   ##
=========================================
- Coverage      89%      82%     -7%     
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  Files         269      266      -3     
  Lines       22063    22009     -54     
=========================================
- Hits        19737    18072   -1665     
- Misses       2326     3937   +1611