Also, why not drop the dict and use an array instead? That way you don’t need to map at all. Add ‘zero’ (or anything really, empty string works too) as the first item and then you can just use the hour as the array’s index:
Edited again to make it even shorter:
{% set word = ['','one','two','three','four','five','six','seven','eight','nine','ten','eleven','twelve'] %}
{%- set index = now().strftime('%-I') | int %}mdi:clock-time-{{word[index]}}
{{- '-outline' if states('sun.sun') != 'above_horizon' }}