Fix RCTConvert analyser error

Summary: @​public

Fix analyser error on RCTConvert where a key used to subscript an
NSMutableDictionary could possibly be nil.

Reviewed By: @alexeylang, @jspahrsummers

Differential Revision: D2498988

React/Base/RCTConvert.m

@@ -422,6 +422,7 @@ RCT_CGSTRUCT_CONVERTER(CGAffineTransform, (@[
     isPackagerAsset = [self BOOL:json[@"__packager_asset"]];
   } else {
     RCTLogConvertError(json, @"an image");
+    return nil;
   }
 
   NSURL *URL = [self NSURL:path];