[BUG FIX] Edge case for Magmoms

[jmmshn]

There's a strange interaction between charge-decorated structures and magmoms.

Charge-decorated structures can site.species.spin == None which would lead to a result of [None, ....] for the _get_magmoms function and then lead to failure when writing the INCAR

[codecov]

Codecov Report

Merging #460 (05f64bd) into main (95b15d5) will decrease coverage by 0.16%.
Report is 5 commits behind head on main.
The diff coverage is 50.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
- Coverage   65.30%   65.15%   -0.16%     
==========================================
  Files          74       74              
  Lines        7186     7186              
  Branches      927      927              
==========================================
- Hits         4693     4682      -11     
- Misses       2208     2220      +12     
+ Partials      285      284       -1     

Files Changed	Coverage Δ
src/atomate2/vasp/sets/base.py	69.23% <0.00%> (ø)
src/atomate2/vasp/sets/defect.py	85.29% <100.00%> (ø)

... and 1 file with indirect coverage changes

[jmmshn]

@utf, @janosh, this was causing failures with structures grabbed with oxidation states.

[utf]

Thanks @jmmshn