Solr does not have a concept of null value. Typically such value would not be indexed at all.

We have at least one field type (MapLocation) that will break indexing if empty. It should be possible to use FieldType::isEmpty() to skip this case, however we have some field types that have indexable empty value, for example TextLine has zero length string as empty value that should probably be indexed.

This case needs to be added to the integration tests.