Converters need updating for Multideterminant wavefunctions with empty spin channel / only some formats work

**Describe the bug**

Report that multideterminant wavefunctions do not work with empty spin channel. This is an important case that in principle should work, but I assume the original protection logic is overly cautious. Has anyone hit this, investigated, or have an alternative route that does work with the current code?

[Full report by Matej on Google Groups including reproducer files](https://groups.google.com/g/qmcpack/c/GctowBQwGj8)

```
I am trying to run a multidet CSF WF for the H_2 triplet, where N_el_down=0

First, for the single-det case, I used the default output from the converter:

-1


-1

This produced the following error:

Fatal Error. Aborting at Some 'group' XML element node doesn't contain a 'size' attribute! 'size = 0' is not allowed in the input. Make appropriate adjustments to the input or converter.

I was able to work around this by simply removing

-1

and

After that, the calculation runs correctly.

However, when I try the multidet version:

........................

I get the following error:

Fatal Error. Aborting at In SlaterDetBuilder: SPOSet "" is not found. Expected for MultiSlaterDeterminant.

I tried:
-removing spo_dn="spo_ud"
-removing ncb="0" and neb="0"
-removing all beta="..."
-using the old WF format

Is there a way to run this, or is the multidet WF not implemented for an empty spin channel?

I attached a complete example reproducing the error. The h2.wfj.xml file also contains a commented-out single-det version.
```

**To Reproduce**
File on google groups.

**Expected behavior**
Runs correctly.

**System:**
N/A