[BUG] fix: rna2vec preserves short sequences as zero rows (closes #363)

[Blackphoenix-15]

Reference Issues/PRs

Fixes #363

What does this implement/fix? Explain your changes.

Removed the if any(converted): guard in rna2vec that silently dropped
sequences shorter than 3 characters. Short sequences now return a zero-padded
row instead, preserving alignment between x_apta, x_prot, and y in APIDataset.

Before: rna2vec(["AAAA", "AA", "CCCC"], max_sequence_length=5) returned shape (2, 5)
After: rna2vec(["AAAA", "AA", "CCCC"], max_sequence_length=5) returns shape (3, 5)

What should a reviewer concentrate their feedback on?

The removal of the if any(converted): block in _rna.py and the dedented
truncate/pad logic that replaces it.

Did you add any tests for the change?

none

Any other comments?

PR checklist

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