Summary:
this is a recent regression because the error appeared in my app (from 0.16 to 0.17).
```
<Image source={{ uri: null }} />
```
will make an error:
```
[RCTConvert.m:55] Error setting property 'source' of RCTImageView with tag #317: JSON value '<null>' of type NSNull cannot be converted to NSString
```
The solution attached is to check that uri is a NSString and return nil if not (the same way nil is returned in case of invalid url).
Closes https://github.com/facebook/react-native/pull/4902
Reviewed By: svcscm
Differential Revision: D2779789
Pulled By: nicklockwood
fb-gh-sync-id: cfe429ec5e53e63b7b368c06e693424ca8aaa11e
f940e7c4a6