OpenUV: null instead of unknown

[metbril]

Home Assistant release with the issue:
0.103.6

Last working Home Assistant release (if known):
n/a

Operating environment (Hass.io/Docker/Windows/etc.):
hassio

Integration:
https://www.home-assistant.io/integrations/openuv/

Description of problem:
When current UV index is 0 (zero), the sun exposure times are defined as null, which renders them as yellow rows in Lovelace. The sensor values should become 'unknown' instead, so the user at least sees a proper sensor in the UI.

Problem-relevant configuration.yaml entries and (fill out even if it seems unimportant):
n/a

Traceback (if applicable):
n/a

Additional information:

[probot-home-assistant]

Hey there @bachya, mind taking a look at this issue as its been labeled with a integration (openuv) you are listed as a codeowner for? Thanks!