Write ignore position if file is not already created

I'm trying to write a file chunk by chunk.
i receive chunks asynchronously so i perform some write with position in this way:

`await RNFS.write(path, payload.toString('base64'), position, {encoding: 'base64'});`

I found a bug into the position handling when the file is not already created. In this case the library write the payload in a file, starting from 0.
If the file is already present (i created it with `writeFile` i.e.) it works as expected, from 0 to position it set 0 and then the payload.

## Sample

```javascript
await RNFS.write(path, 'payload_2', 10);
await RNFS.write(path, 'payload_1 ', 0);
```

### Expected results

`payload_1 payload_2`

### Results

`payload_1 `

the `payload_2` get overridden since it was written into the position 0 instead 10