@Override
    public FieldIsNotNullPredicate<ENTITY, String> isNotNull() {
        return new StringIsNotNullPredicate<>(this);
    }
@param <ENTITY>     the entity type
@param <D>          the database type
@param <FK_ENTITY>  the foreign entity type

@author  Per Minborg
@author  Emil Forslund
@since   2.2.0
runtime-parent/runtime-field/src/main/java/com/speedment/runtime/field/internal/StringForeignKeyFieldImpl.java