解决model里面是NSNull

- (id)mj_newValueFromOldValue:(id)oldValue property:(MJProperty *)property{
   
    if (oldValue == [NSNull null]) {
       
        if ([oldValue isKindOfClass:[NSArray class]]) {
           
            return  @[];
           
        }else if([oldValue isKindOfClass:[NSDictionary class]]){
           
            return @{};
           
        }else{
           
            return @"";
           
        }
       
    }
   
    return oldValue;
   
}