I'm not sure I understand the question. It seems to be related to general database design rather that a Tersus modelling issue.
I think there is an on-going debate on whether or not it is good design to allow a foreign-key field to be set to null, but ultimately most (perhaps all) databases allow defining the field null-able - I do not know where Oracle stands on this issue.
I suggest you first check whether Oracle allows this - perhaps even try a simple sql insert statement on the relevant table using the oracle client. If you're satisfied that the database supports it, but for some reason a Tersus Insert/Update/Database Update fails, please let us know, providing the error message appearing in the application log.
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