Serialiser_Engine: Protect Bson not having an index

While testing https://github.com/BHoM/Versioning_Toolkit/pull/231 locally with @albinber we found a `TargetInvocation` error being thrown https://github.com/BHoM/BHoM_Engine/blob/develop/Serialiser_Engine/Compute/Deserialise/List.cs#L88 here - where the bson object was an array according to bson, but had no elements so `[0]` was throwing an error.

Add a protection to that just in case.