allow `GlmSpeedCallback` without `StepsizeCallback`

Is there a good way to use `RDPK3SpFSAL49()` with the existing MHD solvers? A naive swap results in errors.

For example, running
```julia
trixi_include("examples/unstructured_2d_dgsem/elixir_mhd_ec.jl",
sol = solve(ode, RDPK3SpFSAL49(), abstol=1e-7, reltol=1e-7, save_everystep=false, callback=callbacks))
```
prints the errors
```julia
┌ Warning: First function call produced NaNs. Exiting.
└ @ OrdinaryDiffEq ~/.julia/packages/OrdinaryDiffEq/9xnA1/src/initdt.jl:108
┌ Warning: Automatic dt set the starting dt as NaN, causing instability.
└ @ OrdinaryDiffEq ~/.julia/packages/OrdinaryDiffEq/9xnA1/src/solve.jl:510
┌ Warning: NaN dt detected. Likely a NaN value in the state, parameters, or derivative value caused this outcome.
└ @ SciMLBase ~/.julia/packages/SciMLBase/UIp7W/src/integrator_interface.jl:325
```
I think this is due to the fact that `equations.c_h = NaN` on initialization, and needs `GlmSpeedCallback` to be set properly. This seems to be done when using `CarpenterKennedy2N54` but not `RDPK3SpFSAL49()`.