It's possible to save a blank serial number even when serial number is mandatory.
How to reproduce:
Create a new Attribute Set with serial number and check the field Mandatory serial number
Associate this Attribute Set to a product
Create a new Sales Order, and insert this product to a new line
Click at Attribute Set Instance, create a new Instance with 1234 serial number and save
Now click again at Attribute Set Instance and check Edit record, clean the Serial number and click OK, now iDempiere should throw an error, acknowledge the error and cancel the edit
Apparently the change is ignored, because on Sales Order Line you still see the serial number there, but if you click again at Attribute Set Instance you can check that Serial Number is now blank
Attached you can find the fix.