Fix UpdateSitelinkCampaignExtensionSetting

[PierrickVoulet]

Which version of the client library are you using?
V4.0.0

Which version of PHP are you using?
7.4

Which operating system (Linux, Windows, ...) and version?
Linux

Actions taken
Run the code example UpdateSitelinkCampaignExtensionSetting

Expected result
Successful run of the code example

Actual result
An API request fails:

"errors": [{
  "errorCode": {
    "extensionSettingError": "EXTENSION_SETTING_UPDATE_IS_A_NOOP"
  },
  "message": "The UPDATE operation does not specify any fields other than the resource name in the update mask.",
  "trigger":{
    "stringValue": ""
  },
  "location": {
    "fieldPathElements": [{
      "fieldName": "operations",
      "index": "0"
    }]
  }
}]  

Anything else we should know about your project / environment
It works fine when using the PHP implementation of protobuf but fails when using the C implementation
The difference of behavior is reported on protocolbuffers/protobuf#7650

[PierrickVoulet]

I just tested with protocolbuffers/protobuf#7650 and protocolbuffers/protobuf#7900 and it seems to be fixed. Let's wait for the next protobuf release before closing this though.

[PierrickVoulet]

I am closing the issue because this is fixed with the newer release of protobuf (tested).