Nexus: update how wavefunction is taken by hamiltonian following 5825 input change

Following the input changes introduced in
https://github.com/QMCPACK/qmcpack/pull/5825

Existing files should not break unless errors were detected by the new change.

I think for nexus, just don't set the `wavefunction` attribute unless the simulation is CSVMC.

Not sure what principal nexus follows to generate an QMCPACK input. My recommendation is to write minimal input which is usually compatible with both old and new input scheme.