code: @BsonIgnore @Nullable
      public MongoNamespace getNamespace() {
          if (namespaceDocument == null) {
              return null;
          }
          if (!namespaceDocument.containsKey("db") || !namespaceDocument.containsKey("coll")) {
              return null;
          }
  
          return new MongoNamespace(namespaceDocument.getString("db").getValue(), namespaceDocument.getString("coll").getValue());
      }
docs: Returns the namespace
  
  The invalidate operation type does include a MongoNamespace in the ChangeStreamDocument response. The
  dropDatabase operation type includes a MongoNamespace, but does not include a collection name as part
  of the namespace.
  
  @return the namespace. If the namespaceDocument is null or if it is missing either the 'db' or 'coll' keys,
  then this will return null.