On Windows: "The given path's format is not supported" in mkdir function

Hi everyone, I'm using this module in conjunction with redux-persist-fs-storage (an adapter for redux persist that use this module) and I've encountered this error:


"The given path's format is not supported" in the mkdir function (https://github.com/itinance/react-native-fs/blob/master/windows/RNFS/RNFSManager.cs#L432)

Some info:

```
React Native Environment Info:
System:
OS: Windows 10
CPU: (4) x64 Intel(R) Core(TM) i5-6500 CPU @ 3.20GHz
Memory: 1.68 GB / 7.95 GB
Binaries:
npm: 6.5.0 - C:\Users\Giacomo\AppData\Roaming\npm\npm.CMD
IDEs:
Android Studio: Version 3.2.0.0 AI-181.5540.7.32.5056338
```

React-native-fs successfully linked and v.2.13.3