The row with null is not persisted,
if you check
https://github.com/hibernate/hibernate-orm/blob/master/hibernate-core/src/main/java/org/hibernate/persister/collection/AbstractCollectionPersister.java#L1280
the collection element is added to the PreparedStatement only if

collection.entryExists( entry, i ) == true

but

PersistenMap#entryExists

implementation is

public boolean entryExists(Object entry, int i) {
	return ( (Map.Entry) entry ).getValue() != null;
}

returning false in case the map value is null.