fix bug regex silence will silence all alert

[wanjunlei]

Fixed bug where regex silence would silence alerts that did not have the match expression key label

For example, all alerts without the label abc will be silenced by the following silence, this is not expected behavior.

apiVersion: notification.kubesphere.io/v2beta2
kind: Silence
metadata:
  name: silence1
  labels:
    type: global
spec:
  matcher:
    matchExpressions:
      - key: abc
        operator: Match
        regexValue: ".*abc"
  startsAt: "2022-02-24T01:15:20Z"
  schedule: ""

[benjaminhuo]

发个 2.5.1？