Set empty fields to NULL instead of an empty object

[lazappi]

Currently, when an object is created any empty fields are filled with an empty object of the expected class. For example:

db <- connect("laminlabs/cellxgene")
artifact <- db$Artifact$get("KBW89Mf7IGcekja2hADu")
run <- artifact$storage$run

Here run is an empty Run object. Instead we should set it to NULL which would be cleaner and match Python.

[lazappi]

Should have mentioned you get an error if you try to access things in the empty object:

> run$id
Error in `private$.api$get_record()` at laminr/R/Record.R:234:9:
! Method Not Allowed

Possibly there is another solution to this but avoiding the empty objects in the first place has other advantages