Does the emission of a small amount of semen after performing ghusl from masturbation, just before the dawn athan, affect the validity and acceptance of the fast?

Masturbation before dawn does not invalidate the fast, nor does the emission of residual semen after dawn, due to nocturnal masturbation, invalidate the fast. Your fast is valid, and no compensatory fast is due from you. It is preferable to perform ghusl (ritual bath) for any residual semen that is emitted after the initial ghusl.