[FIX] C++26: std::optional is a view

[eseiler]

[ RUN      ] pretty_printing.optional
../../../src/test/unit/test/pretty_printing_test.cpp:71: Failure
Expected equality of these values:
  debug_str(std::optional<int>{})
    Which is: "[]"
  "<VALUELESS_OPTIONAL>"s
    Which is: "<VALUELESS_OPTIONAL>"

score_matrix: 9 columns and 5 rows
 ;    ;    ;    ;    ;    ;    ;    ;    ;    ;
 ;[0] ;[-1];[-2];[-3];[-4];[-5];[-6];[-7];[-8];
 ;[-1];[0] ;[-1];[-2];[-3];[-4];[-5];[-6];[-7];
 ;[-2];[-1];[-1];[-1];[-2];[-3];[-4];[-5];[-6];
 ;[-3];[-2];[-2];[-2];[-2];[-2];[-3];[-4];[-5];
 ;[-4];[-3];[-3];[-3];[-3];[-3];[-3];[-3];[-4];

vs

score_matrix: 9 columns and 5 rows
 ;  ;  ;  ;  ;  ;  ;  ;  ;  ;
 ;0 ;-1;-2;-3;-4;-5;-6;-7;-8;
 ;-1;0 ;-1;-2;-3;-4;-5;-6;-7;
 ;-2;-1;-1;-1;-2;-3;-4;-5;-6;
 ;-3;-2;-2;-2;-2;-2;-3;-4;-5;
 ;-4;-3;-3;-3;-3;-3;-3;-3;-4;

[seqan-actions]

Documentation preview available at https://docs.seqan.de/preview/seqan/seqan3/3368

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.09%. Comparing base (74a7ed6) to head (616123c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3368   +/-   ##
=======================================
  Coverage   98.09%   98.09%           
=======================================
  Files         270      270           
  Lines       11260    11260           
  Branches      105      105           
=======================================
  Hits        11046    11046           
  Misses        214      214           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.