Fix one warning: expected ‘struct NSError **’ but argument is of type ‘id’

nil is not recognized by GCC as a (NSError**), but as an (id).  NULL works the
same but does not spawn warnings.

SoObjects/Mailer/SOGoDraftObject.m

@@ -1116,7 +1116,7 @@ static NSString    *userAgent      = nil;
       pmime = [self pathToAttachmentWithName: [NSString stringWithFormat: @".%@.mime", name]];
       if (![[mimeType dataUsingEncoding: NSUTF8StringEncoding] writeToFile: pmime atomically: YES])
         {
-          [[NSFileManager defaultManager] removeItemAtPath: p error: nil];
+          [[NSFileManager defaultManager] removeItemAtPath: p error: NULL];
           return [NSException exceptionWithHTTPStatus: 500 /* Server Error */
                                                reason: @"Could not write attachment to draft!"];
         }