Fix undefined variable in openmc.mgxs.library (scatt_mgxs)

[hugo-barthod]

Description

In openmc.mgxs.library, lines 1286-1292 suggest it okay to build a multigroup library from only a scatter matrix (ie without multiplicity being accounted for).
However when reaching these lines, scatt_mgxs was undefined variable and raised error.
This PR defines scatt_mgxs in this case.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks @hugo-barthod!