Indexing with dspace index-discovery -b fails

[olli-gold]

Describe the bug
When executing bin/dspace index-discovery -b the command fails nearly immediately on our system with a NullPointerException. This occurs on DSpace-CRIS 2023.02.05.

The NullPointer is located in the attribute mappableMetadata here: https://github.com/4Science/DSpace/blob/main-cris/dspace-api/src/main/java/org/dspace/discovery/SolrServiceFileInfoPlugin.java#L189. The file has been modified in commit f0ed448, so the issue has probably been introduced in DSpace-CRIS 2023.02.03.

To Reproduce
Steps to reproduce the behavior:

1. Start bin/dspace index-discovery -b via CLI

Expected behavior
The indexing should work without the exception.

Related work
I will create a PR fixing this and link it in the comments.