fix: ignore protobuf files without messages when generating code

[infastin]

fastmarshal at the moment generates empty files if a protobuf file doesn't contain any messages.

So, for this file:

syntax = "proto3";

package nats;

import "google/protobuf/descriptor.proto";

option go_package = "natspb";

extend google.protobuf.FieldOptions {
  int32 subject = 123;
}

fastmarshal generates this:

// GENERATED CODE - DO NOT EDIT
// This file was generated by protoc-gen-fastmarshal

package natspb

import (
	"fmt"
	"sync/atomic"
	"github.com/CrowdStrike/csproto"
)

This PR makes it so fastmarshal first checks if there are any messages in a protobuf file before generating the code. grpc and drpc generators do the same thing.

[wmorgan6796]

Overall looks good. There are 2 things:

1. It feels like there needs to be a test case added somewhere for this (I'm not entirely sure where though) to validate the new behavior.
2. There is a typo (that was there from the moved function before) that should be fixed.

[infastin]

Added a test to cmd/protoc-gen-fastmarshal.

[wmorgan6796]

Looks Good! thank you for the contribution!!