Allow zero purge_interval to disable recorder purge

[amelchio]

Description:

This was suggested in #11976 (comment) as an easy way to disable recorder purge, now that purging soon becomes the default.

It turns out that we still have the code to disable purging if purge_interval is falsy. It was a leftover from when purge_interval could be None so maybe I subconsciously anticipated this suggestion :)

So this PR just includes zero in the allowed range.

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#4591

Example entry for configuration.yaml (if applicable):

recorder:
  purge_interval: 0

Checklist:

• The code change is tested and works locally.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code does not interact with devices:

• Local tests with tox run successfully.
• Tests have been added to verify that the new code works.