Fix default constraints in Manopt.jl (nothing -> missing in v0.6.4)

[JoshuaLampert]

In JuliaManifolds/Manopt.jl#630 the default for g, grad_g, h, and grad_h changed from nothing to missing, i.e., explicitly passing nothing does not work anymore with Manopt.jl >=v0.6.4. Since it is the default anyway, simply removing the kwargs should fix it on both >=v0.6.4 and <v0.6.4.

[JoshuaLampert]

There were some issues with Manopt.jl v0.6.4 and v0.6.5, which should hopefully be fixed with Manopt.jl v0.6.6.

[JoshuaLampert]

Ok, now the registry server is back and we get Manopt.jl v0.6.6., but there are new failures. This time they seem to come from trixi-framework/TrixiBase.jl#76.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.20%. Comparing base (cd1809d) to head (be4f2e9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #100   +/-   ##
=======================================
  Coverage   98.20%   98.20%           
=======================================
  Files          26       26           
  Lines        1389     1389           
=======================================
  Hits         1364     1364           
  Misses         25       25           

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