Generator fails when `./build/rdgen/.rdgen` doesn't exist

```
>> Generating: 2 filtered generators
Invoke CSharp50Generator(defaultFlowTransform=Reversed, defaultNamespace='JetBrains.Rider.Model', folder=/Users/eugene/Developer/resharper-fsharp/ReSharper.FSharp/src/FSharp.ProjectModelBase/src/Protocol) on root `IdeRoot`, clearFolder=false
Invoke Kotlin11Generator(flowTransform=AsIs, defaultNamespace='com.jetbrains.rider.model', folder=/Users/eugene/Developer/resharper-fsharp/rider-fsharp/src/main/java/com/jetbrains/rider/plugins/fsharp/protocol) on root `IdeRoot`, clearFolder=false
Generation finished in 423 ms
Storing hash into 'build/rdgen/.rdgen'

> Task :rdgen FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':rdgen'.
> java.io.FileNotFoundException: build/rdgen/.rdgen (No such file or directory)
```

A workaround is creating an empty `./build/rdgen/.rdgen` file.

Is there some additional information I can provide?

```
eugene@Eugenes-MacBook:rider-fsharp$ java --version
java 12.0.2 2019-07-16
Java(TM) SE Runtime Environment (build 12.0.2+10)
Java HotSpot(TM) 64-Bit Server VM (build 12.0.2+10, mixed mode, sharing)
```