protoc-gen-go-grpc: empty const block causing `gofmt` failure

[conorevans]

What version of gRPC are you using?

✗ protoc-gen-go --version
protoc-gen-go v1.28.1
✗ protoc --version
libprotoc 3.19.4

What version of Go are you using (go version)?

✗ go version
go version go1.20.5 darwin/arm64

What operating system (Linux, Windows, …) and version?

macOS 14.4 (23E214)

What did you do?

Generate Go protos off the following empty service definition

syntax = "proto3";
package foobar;
option go_package = "github.com/foobar";

service Foobar {}

which leads to

// Code generated by protoc-gen-go-grpc. DO NOT EDIT.
// versions:
// - protoc-gen-go-grpc v1.3.0
// - protoc             v4.25.3
// source: service-foobar.proto

package popsa

import (
	grpc "google.golang.org/grpc"
)

// This is a compile-time assertion to ensure that this generated file
// is compatible with the grpc package it is being compiled against.
// Requires gRPC-Go v1.32.0 or later.
const _ = grpc.SupportPackageIsVersion7

const ()

// FoobarClient is the client API for Foobar service.
//
// For semantics around ctx use and closing/ending streaming RPCs, please refer to https://pkg.go.dev/google.golang.org/grpc/?tab=doc#ClientConn.NewStream.
type FoobarClient interface {
}

What did you expect to see?

No gofmt error

What did you see instead?

gofmt error due to empty const block (since no methods are defined)

---

I am aware that an empty service proto definition is a bit pointless, but I dealt with a broken workflow due to it today, and figured I'd fix it in the generator