Fixed: CPNumberFormatter formatted @"" as any number.

Without this fix, `- CPNumberFormatter getObjectValue:forString:errorDescription:` would return `YES` for string @"", like if it was properly converted to an object value, but then actually leave object value unchanged.

With this fix object value is set to nil like in Cocoa.

Refs #1829.

Foundation/CPNumberFormatter.j

@@ -183,9 +183,12 @@ var NumberRegex = new RegExp('(-)?(\\d*)(\\.(\\d*))?');
 
 - (BOOL)getObjectValue:(id)anObjectRef forString:(CPString)aString errorDescription:(CPString)anErrorRef
 {
-    // allows an empty string to pass without validation
+    // Interpret an empty string as nil, like in Cocoa.
     if (aString === @"")
+    {
+        @deref(anObjectRef) = nil;
         return YES;
+    }
 
     var value = [self numberFromString:aString],
         error = @"";