Document with optional partition key

I have 4 collections in my databsse, 3 with partition key\value and 1 collection without partition key ( defined as optional )
I can sync in my app ( android ) on the 3 collection using a partition value.
The collection without partition value - according to the docs, should go into “null” partition .
I have tried opening another “realm” with “null” partition , but I not getting any documents when doing so.

Is there an example for using a partition key as optional and sync on “null” partition ? Any suggestions ?