Avoid scikit-learn UserWarning for vectorizer parameter token_pattern

[osma]

scikit-learn vectorizers used by Annif (CountVectorizer, TfidfVectorizer) trigger this warning:

UserWarning: The parameter 'token_pattern' will not be used since 'tokenizer' is not None'

This is a bit surprising, since we are not setting token_pattern ourselves, but its default value is not None. This PR fixes the warning by explicitly setting token_pattern=None whenever the tokenizer parameter is set in Annif calling code.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (40cc2fd) 99.67% compared to head (a84e466) 99.67%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #729   +/-   ##
=======================================
  Coverage   99.67%   99.67%           
=======================================
  Files          89       89           
  Lines        6397     6401    +4     
=======================================
+ Hits         6376     6380    +4     
  Misses         21       21           

Files Changed	Coverage Δ
annif/lexical/mllm.py	100.00% <ø> (ø)
annif/backend/mixins.py	97.82% <100.00%> (+0.20%)	⬆️

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication