Add test coverage in TestGroupKeyManagementCluster for EpochStartTime0 set to 0

[bzbarsky-apple]

See #26726 (comment)

@rcasallas-silabs could you figure out what needs testing here?

[lpbeliveau-silabs]

Closing this because TestGroupKeyManagement has a step that test this:

• label:
  "KeySetWrite with EpochStartTime0 set to zero fails INVALID_COMMAND"
  command: "KeySetWrite"
  arguments:
  values:
  - name: "GroupKeySet"
  value:
  {
  GroupKeySetID: 0x01a1,
  GroupKeySecurityPolicy: 0,
  EpochKey0: "\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf",
  EpochStartTime0: 0,
  EpochKey1: null,
  EpochStartTime1: null,
  EpochKey2: null,
  EpochStartTime2: null,
  }
  response:
  error: INVALID_COMMAND