Make PrometheusRule.spec.groups[].rules optional

[mcbenjemaa]

Description

This PR makes PrometheusRule.spec.groups[].rules optional,
so, a manifest without rules defined can be accepted.

apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  name: example
spec:
  groups:
  - name: example
    rules:

Type of change

What type of changes does your code introduce to the Prometheus operator? Put an x in the box that apply.

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Changelog entry

Please put a one-line changelog entry below. This will be copied to the changelog file during the release process.

Change field `PrometheusRule.spec.groups[*].rules` to optional

Closes #5480

[slashpai]

Looks like a PR already created for this #5481

[mcbenjemaa]

Oh, didn’t know that

[slashpai]

Closing in favour of #5481