[BUG]: root collection ID should be null if empty string (#4663)

## Description of changes

Some rows in our database have `root_collection_id: ""` instead of
having it set to null. We're not sure why this happened, this is a quick
patch as a short-term fix while we continue to investigate.

## Test plan

_How are these changes tested?_

- [ ] Tests pass locally with `pytest` for python, `yarn test` for js,
`cargo test` for rust

## Documentation Changes

_Are all docstrings for user-facing APIs updated if required? Do we need
to make documentation changes in the [docs
section](https://github.com/chroma-core/chroma/tree/main/docs/docs.trychroma.com)?_

Author: codetheweb

go/pkg/sysdb/metastore/db/dao/collection.go

@@ -30,7 +30,7 @@ func (s *collectionDb) DeleteAll() error {
 func (s *collectionDb) GetCollectionWithoutMetadata(collectionID *string, databaseName *string, softDeletedFlag *bool) (*dbmodel.Collection, error) {
 	var collections []*dbmodel.Collection
 	query := s.db.Table("collections").
-		Select("collections.id, collections.name, collections.database_id, collections.is_deleted, collections.tenant, collections.version, collections.version_file_name, collections.root_collection_id, NULLIF(collections.lineage_file_name, '') AS lineage_file_name").
+		Select("collections.id, collections.name, collections.database_id, collections.is_deleted, collections.tenant, collections.version, collections.version_file_name, NULLIF(collections.root_collection_id, '') AS root_collection_id, NULLIF(collections.lineage_file_name, '') AS lineage_file_name").
 		Joins("INNER JOIN databases ON collections.database_id = databases.id").
 		Where("collections.id = ?", collectionID)