BUG: aims parser for spin coll. calculation + MD_light

The FHI-aims parser throws an error when parsing a spin collinear calculation with MD_light outputs. This MR fixes the issue.

The behavior was previously untested, not sure if it is severe enough to justify adding a dedicated test.

Pinging @tpurcell90 and @Kokookster

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.