fix: missing unk, fix bug

[stephantul]

The unk was missing and I also introduced a huge bug, both are fixed.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
model2vec/tokenizer/tokenizer.py	64.28%	5 Missing ⚠️

Files with missing lines	Coverage Δ
model2vec/tokenizer/model.py	100.00% <100.00%> (ø)
model2vec/tokenizer/tokenizer.py	90.41% <64.28%> (-2.09%)	⬇️

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