Fix LightningCLI optional callback handling in subclass mode

[KAVYANSHTYAGI]

When add_lightning_class_args(..., subclass_mode=True, required=False) is used for callbacks and no CLI argument is provided, the resolved value is None.

Previously, LightningCLI.instantiate_trainer unconditionally appended these values to the Trainer callbacks list, allowing None entries to silently propagate.

This PR filters out None callback entries before passing them to the Trainer, preserving the invariant that Trainer.callbacks contains only Callback instances.

A regression test is added to cover optional subclass-mode callback arguments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (215d7d9) to head (9d55321).
✅ All tests successful. No failed tests found.

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HEAD has 429 uploads less than BASE

Flag	BASE (215d7d9)	HEAD (9d55321)
cpu	132	33
python	12	3
lightning_fabric	36	0
pytest	66	0
python3.10	12	3
lightning	60	15
python3.12	36	9
python3.13	12	3
python3.11	24	6
python3.12.7	36	9
pytorch2.8	12	6
pytorch_lightning	36	18
pytest-full	66	33
pytorch2.6	6	3
pytorch2.5.1	6	3
pytorch2.9	6	3
pytorch2.7	6	3
pytorch2.2.2	6	3
pytorch2.1	12	6
pytorch2.4.1	6	3
pytorch2.3	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21527     +/-   ##
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- Coverage      87%      79%     -8%     
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  Files         270      267      -3     
  Lines       24069    24018     -51     
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- Hits        20868    18962   -1906     
- Misses       3201     5056   +1855