Please provide the following (and please check them off the list with [x]) before submitting this issue:
Expected behavior
When initializing a new list item object we provide 'null' for some fields if the source is empty or null:

Actual behavior
When initializing a new list item object we provide 'null' for some fields if the source is empty or null:

When submitting the newly created listitem to the GraphServiceClient:

This results in a Generic Service Exception, after investigating I realized this had to do with the field data types and there respective option to allow the field to have no content.
But since this week, it is no longer allowed to send null values using the MsGraph.
Steps to reproduce the behavior
Prepare a listitem object like in Expected Behavior.
Then try to add it using the MsGraph Core SDK