Fix no output when Client=False,TestClient=True (#1765)

Sources/protoc-gen-grpc-swift/main.swift

@@ -156,7 +156,9 @@ func main(args: [String]) throws {
         binaryFile.content = serializedFileDescriptorProto
         response.file.append(binaryFile)
       }
-      if !fileDescriptor.services.isEmpty && (options.generateClient || options.generateServer) {
+      if !fileDescriptor.services.isEmpty
+        && (options.generateClient || options.generateServer || options.generateTestClient)
+      {
         var grpcFile = Google_Protobuf_Compiler_CodeGeneratorResponse.File()
         let grpcFileName = uniqueOutputFileName(
           component: "grpc",