Fix default zeta in struct definition

[jClugstor]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[github-actions]

Benchmark Results

Click to check benchmark results

	master	b8813ea...	master / b8813ea...
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK2()	0.583 ± 0.019 s	0.587 ± 0.02 s	0.992 ± 0.046
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK3()	12.6 ± 0.6 ms	12.3 ± 0.51 ms	1.02 ± 0.065
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK4()	2.44 ± 0.23 ms	2.36 ± 0.13 ms	1.04 ± 0.11
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK5()	2.93 ± 0.52 ms	2.94 ± 0.53 ms	0.996 ± 0.25
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK6()	1.14 ± 0.25 ms	1.16 ± 0.25 ms	0.986 ± 0.3
Simple Pendulum/IIP/MultipleShooting(10, Tsit5; grid_coarsening = false)	2.05 ± 0.75 ms	2.05 ± 0.71 ms	1 ± 0.5
Simple Pendulum/IIP/MultipleShooting(10, Tsit5; grid_coarsening = true)	4.25 ± 1.2 ms	4.26 ± 1.2 ms	0.996 ± 0.4
Simple Pendulum/IIP/MultipleShooting(100, Tsit5; grid_coarsening = false)	0.0545 ± 0.0063 s	0.0536 ± 0.0064 s	1.02 ± 0.17
Simple Pendulum/IIP/MultipleShooting(100, Tsit5; grid_coarsening = true)	0.0863 ± 0.0074 s	0.0822 ± 0.0097 s	1.05 ± 0.15
Simple Pendulum/IIP/Shooting(Tsit5())	0.258 ± 0.087 ms	0.242 ± 0.085 ms	1.06 ± 0.52
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK2()	0.764 ± 0.031 s	0.754 ± 0.015 s	1.01 ± 0.045
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK3()	15.9 ± 7.1 ms	15.6 ± 6.2 ms	1.02 ± 0.61
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK4()	2.85 ± 0.21 ms	2.78 ± 0.17 ms	1.03 ± 0.099
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK5()	3.57 ± 0.3 ms	3.39 ± 0.18 ms	1.05 ± 0.11
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK6()	1.35 ± 0.17 ms	1.32 ± 0.12 ms	1.03 ± 0.16
Simple Pendulum/OOP/MultipleShooting(10, Tsit5; grid_coarsening = false)	4.17 ± 3.3 ms	3.78 ± 3.2 ms	1.1 ± 1.3
Simple Pendulum/OOP/MultipleShooting(10, Tsit5; grid_coarsening = true)	8.04 ± 7.3 ms	7.54 ± 6.2 ms	1.07 ± 1.3
Simple Pendulum/OOP/MultipleShooting(100, Tsit5; grid_coarsening = false)	0.116 ± 0.0077 s	0.107 ± 0.011 s	1.08 ± 0.13
Simple Pendulum/OOP/MultipleShooting(100, Tsit5; grid_coarsening = true)	0.168 ± 0.034 s	0.165 ± 0.01 s	1.02 ± 0.21
Simple Pendulum/OOP/Shooting(Tsit5())	0.612 ± 0.074 ms	0.607 ± 0.051 ms	1.01 ± 0.15
time_to_load	7.05 ± 0.048 s	7.09 ± 0.11 s	0.993 ± 0.017

### Benchmark Plots A plot of the benchmark results has been uploaded as an artifact to the workflow run for this PR. Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[ChrisRackauckas]

Why not allow nothing in the type here? @ErikQQY