I did not see the small circle used to indicate niente at the end (or start) of a hairpin in the palette, but discovered that if I click the hairpin and then the word niente, Dorico will create the symbols (see image). I am sure this is old news for many of you, but new users–or people like me who have just not ever needed it before–may not know.
I see that this is also possible in the dynamics popover.
