pb: return empty server address for malformed grpc addresses

GrpcAddressToServerAddress used to return the unparseable input on a
hostAndPort failure, so a malformed raft address (e.g. "host:abc")
flowed into admin dashboard master maps unchanged. Return an empty
string instead, skip empty conversions at the two raft-cluster merge
sites, and drop the now-stale comment about the fatal exit the earlier
commit removed.

weed/admin/dash/admin_server.go

@@ -1351,6 +1351,9 @@ func (s *AdminServer) GetClusterMasters() (*ClusterMastersData, error) {
 		for _, server := range resp.ClusterServers {
 			// Raft stores gRPC addresses, convert to HTTP address
 			httpAddress := pb.GrpcAddressToServerAddress(server.Address)
+			if httpAddress == "" {
+				continue
+			}
 
 			// Update existing master info or create new one
 			if masterInfo, exists := masterMap[httpAddress]; exists {