Handle RPC methods with missing options

[alexpearce]

Given the following file priv/test.proto:

import "google/api/annotations.proto";

service TestService {
  rpc Get(TestServiceGetRequest)
    returns (stream TestServiceGetResponse) {
    option (google.api.http) = {
      post: "/v1/test"
      body: "*"
    };
  };

  rpc GetOther(TestServiceGetOtherRequest)
    returns (stream TestServiceGetOtherResponse);
}

message TestServiceGetRequest {}
message TestServiceGetResponse {}

message TestServiceGetOtherRequest {}
message TestServiceGetOtherResponse {}

This command will fail as of 0.2.0:

mix protobuf.generate \
  --generate-descriptors=false \
  --include-path=deps/googleapis \
  --include-path=priv
  --plugin=ProtobufGenerate.Plugins.GRPCWithOptions
  --output-path=tmp \
  test.proto

Giving:

** (FunctionClauseError) no function clause matching in ProtobufGenerate.Plugins.GRPCWithOptions.opts/1

    The following arguments were given to ProtobufGenerate.Plugins.GRPCWithOptions.opts/1:

        # 1
        nil

    Attempted function clauses (showing 2 out of 2):

        defp opts(%Google.Protobuf.MethodOptions{__pb_extensions__: extensions}) when extensions == %{}
        defp opts(%Google.Protobuf.MethodOptions{__pb_extensions__: extensions})

    (protobuf_generate 0.2.0) lib/protobuf_generate/plugins/grpc_with_options.ex:79: ProtobufGenerate.Plugins.GRPCWithOptions.opts/1
    (protobuf_generate 0.2.0) lib/protobuf_generate/plugins/grpc_with_options.ex:58: anonymous fn/2 in ProtobufGenerate.Plugins.GRPCWithOptions.generate/2
    (elixir 1.18.4) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
    (elixir 1.18.4) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
    (protobuf_generate 0.2.0) lib/protobuf_generate/plugins/grpc_with_options.ex:52: anonymous fn/3 in ProtobufGenerate.Plugins.GRPCWithOptions.generate/2
    (elixir 1.18.4) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
    (protobuf_generate 0.2.0) lib/protobuf_generate/code_gen.ex:24: anonymous fn/3 in ProtobufGenerate.CodeGen.generate/3
    (elixir 1.18.4) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2

The command passes if the GetOther method is given an empty method options block:

rpc GetOther(TestServiceGetOtherRequest)
  returns (stream TestServiceGetOtherResponse) {};

It seems reasonable to me for a service definition to have some methods with options blocks and some without, so this PR modifies the GRPCWithOptions plugin to allow for missing options blocks.

This PR lets the command above succeed given the original priv/test.proto file. The tmp/test.pb.ex file that results is:

defmodule TestServiceGetRequest do
  @moduledoc false
  use Protobuf, protoc_gen_elixir_version: "0.14.0", syntax: :proto2
end

defmodule TestServiceGetResponse do
  @moduledoc false
  use Protobuf, protoc_gen_elixir_version: "0.14.0", syntax: :proto2
end

defmodule TestServiceGetOtherRequest do
  @moduledoc false
  use Protobuf, protoc_gen_elixir_version: "0.14.0", syntax: :proto2
end

defmodule TestServiceGetOtherResponse do
  @moduledoc false
  use Protobuf, protoc_gen_elixir_version: "0.14.0", syntax: :proto2
end

defmodule TestService.Service do
  @moduledoc false
  use GRPC.Service, name: "TestService", protoc_gen_elixir_version: "0.14.0"

  rpc(:Get, TestServiceGetRequest, stream(TestServiceGetResponse), %{})

  rpc(:GetOther, TestServiceGetOtherRequest, stream(TestServiceGetOtherResponse), %{})
end

defmodule TestService.Stub do
  @moduledoc false
  use GRPC.Stub, service: TestService.Service
end