OCPBUGS-28590: gcp: better error msg when service accnt missing

The JSON object is not set when the credentials are loaded from the cli,
resulting in an unhelpful message about "unexpected end of JSON input".
Let's provide a better error message in that case.

Author: r4f4

pkg/asset/machines/gcp/machines.go

@@ -168,6 +168,12 @@ func provider(clusterID string, platform *gcp.Platform, mpool *gcp.MachinePool,
 				return nil, err
 			}
 
+			// The JSON can be `nil` if auth is provided from env
+			// https://pkg.go.dev/golang.org/x/[email protected]/google#Credentials
+			if len(sess.Credentials.JSON) == 0 {
+				return nil, fmt.Errorf("could not extract service account from loaded credentials. Please specify a service account to be used for shared vpc installations in the install-config.yaml")
+			}
+
 			var found bool
 			serviceAccount := make(map[string]interface{})
 			err = json.Unmarshal(sess.Credentials.JSON, &serviceAccount)