Initialize complex types to NaN for both components

[WardBrian]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] OR, no user-facing changes were made

Release notes

This tweaks code gen slightly to initialize empty complex Eigen types to have nan for both the real and imaginary portions.

Currently, the model:

transformed data {
   complex_vector[1] Z;
}

model {
   print(Z[1]);
}

prints (nan, 0). This PR makes that (nan, nan)

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[bob-carpenter]

+1 to this fix. I should've caught this the first time around.

[rok-cesnovar]

Thanks, looks good to me.