Add empty proto file test

* Add empty proto file test
* Add a preamble to `foo-messages.proto`
* Don't unconditionally add a dependency on `GRPCProtobuf` if there are
  no services
* Add the new test case code generation to `dev/protos/generate.sh`
* depend on `grpc-swift` `main` to pick up empty file generation
  changes.

Package.swift

@@ -31,7 +31,7 @@ let products: [Product] = [
 let dependencies: [Package.Dependency] = [
   .package(
     url: "https://github.com/grpc/grpc-swift.git",
-    exact: "2.0.0-beta.2"
+    branch: "main"
   ),
   .package(
     url: "https://github.com/apple/swift-protobuf.git",

Sources/GRPCProtobufCodeGen/ProtobufCodeGenParser.swift

@@ -98,9 +98,11 @@ package struct ProtobufCodeGenParser {
 
 extension ProtobufCodeGenParser {
   fileprivate func codeDependencies(file: FileDescriptor) -> [Dependency] {
-    var codeDependencies: [Dependency] = [
-      Dependency(module: "GRPCProtobuf", accessLevel: .internal)
-    ]
+    var codeDependencies: [Dependency] = []
+
+    if file.services.count > 0 {
+      codeDependencies.append(Dependency(module: "GRPCProtobuf", accessLevel: .internal))
+    }
 
     // If there's a dependency on a bundled proto then add the SwiftProtobuf import.
     //

dev/protos/generate.sh

@@ -119,6 +119,17 @@ function generate_foo_service_descriptor_set {
     --include_imports
 }
 
+function generate_foo_messages_descriptor_set {
+  local proto proto_path output
+  proto="$here/local/foo-messages.proto"
+  proto_path="$(dirname "$proto")"
+  output="$root/Tests/GRPCProtobufCodeGenTests/Generated/foo-messages.pb"
+
+  invoke_protoc --descriptor_set_out="$output" "$proto" -I "$proto_path" \
+    --include_source_info \
+    --include_imports
+}
+
 function generate_bar_service_descriptor_set {
   local proto proto_path output
   proto="$here/local/bar-service.proto"
@@ -152,5 +163,6 @@ generate_error_service
 # Descriptor sets for tests
 generate_test_service_descriptor_set
 generate_foo_service_descriptor_set
+generate_foo_messages_descriptor_set
 generate_bar_service_descriptor_set
 generate_wkt_service_descriptor_set

dev/protos/local/foo-messages.proto

@@ -1,3 +1,4 @@
+// Leading trivia.
 syntax = "proto3";
 
 package foo;