“Niente” (“nothing”) is used to start a crescendo from silence or end a diminuendo with silence.
Other scores abbreviate it as “n.” in italic typeface (“Diagon Alley” score reduction on YouTube):
I did some looking around just now but I couldn’t find any rules, guidelines or other references to which manner of abbreviating it is considered more correct. The few scores I have that abbreviate it use n., but most of those are film scores, which use some specific conventions anyway, so I hesitate to draw conclusions from that.
Is there any reason – logical, historical, etc. – to prefer either n. or n, or is it up to personal preference?