Minor cleanup of some OSM classes

[habrahamsson-skanetrafiken]

Summary

Small cleanup. Pure refactoring.

• Add @Nullable to some nullable return values.
• generateI18NForPattern() will never be called with a null value in practice, so I removed the null check.
• getTagsByPrefix() didn't need to return null so I removed that case.

Issue

See #6048

Unit tests

No tests for this.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.85%. Comparing base (07c72ea) to head (1385e4d).
Report is 3 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...entripplanner/openstreetmap/model/OSMWithTags.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #6129   +/-   ##
==========================================
  Coverage      69.85%   69.85%           
+ Complexity     17497    17496    -1     
==========================================
  Files           1976     1976           
  Lines          74710    74705    -5     
  Branches        7645     7643    -2     
==========================================
- Hits           52191    52188    -3     
- Misses         19868    19869    +1     
+ Partials        2651     2648    -3     

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