.Net MEVD Weaviate: support null indexing

In Weaviate, filtering by null-state requires special indexing to be specified in the collection definition; tests currently fail with:

Error occurred during vector search. Response: {"data":{"Get":{"FilterTests":null}},"errors":[{"locations":[{"column":5,"line":3}],"message":"explorer: get class: concurrentTargetVectorSearch): explorer: get class: vector search: object vector search at index filtertests: shard filtertests_7iX7kMpaSItW: build inverted filter allow list: fetch doc ids for prop/value pair: Nullstate must be indexed to be filterable! Add `indexNullState: true` to the invertedIndexConfig","path":["Get","FilterTests"]}]}

Supporting this looks it would require us to have a provider-specific schema extensibility (#10359).