Modeling Optional Foreign Key from Composite Primary Key field (Oracle Data Modeler)
You have the foreign key in the wrong table. Your defining entity is in the MKT_DATA_BONDS table. The refering entity is in the BONDS table. Each BOND must be defined in MKT_DATA_BONDS. The "optional" part is carried out by the fact that not all market bonds need be represented as a company bond.

To put it in OO language, market bond is the super or parent class. A company bond is one (of possibly several) sub or derived classes. Subclasses know about super classes, but the super class doesn't know about (or refer to) any subclass.