FIX Allow epoched EEGLAB files without events

[viranovskaya]

Closes #6610.

Some epoched EEGLAB files contain neither EEG.epoch nor EEG.event, for example after the original events fall outside a cropped epoch. read_epochs_eeglab currently fails on these files.

This change:

• warns that the event information is missing;
• assigns every epoch to a single unknown event category;
• uses sequential sample indices for the synthetic events;
• leaves the existing event-reading path unchanged when metadata is present.

The regression test creates synthetic .set files with both empty and absent event fields and checks the loaded data, events, and event ID.

Local checks:

• EEGLAB test module available without the external testing dataset: 2 passed, 26 skipped
• ruff check and format
• ty
• codespell
• rstcheck

[larsoner]

Looks reasonable, thanks @viranovskaya !