この問題は RestKit だけに限定されているとは思いません。JSONを提供しているサーバーからの詳細。私は AFNetworking を使用して同じ問題を抱えています。
Null を削除して空の文字列に置き換えるために使用できる、この Dictionary カテゴリを見つけました。それを使用するか、単にロジックを取り出して null 値を並べ替えることができます。
@implementation NSDictionary (JRAdditions)
- (NSDictionary *) dictionaryByReplacingNullsWithStrings {
    NSMutableDictionary *replaced = [NSMutableDictionary dictionaryWithDictionary:self];
    const id nul = [NSNull null];
    const NSString *blank = @"";
    for(NSString *key in self) {
        const id object = [self objectForKey:key];
        if(object == nul) {
            //pointer comparison is way faster than -isKindOfClass:
            //since [NSNull null] is a singleton, they'll all point to the same
            //location in memory.
            [replaced setObject:blank forKey:key];
        }
    }
    return [NSDictionary dictionaryWithDictionary:replaced];
}
+ (NSDictionary *)dictionaryByReplacingNullsWithStrings:(NSDictionary *)dict {
    NSMutableDictionary *replaced = [NSMutableDictionary dictionaryWithDictionary:dict];
    const id nul = [NSNull null];
    const NSString *blank = @"";
    for(NSString *key in dict) {
        const id object = [dict objectForKey:key];
        if(object == nul) {
            //pointer comparison is way faster than -isKindOfClass:
            //since [NSNull null] is a singleton, they'll all point to the same
            //location in memory.
            [replaced setObject:blank forKey:key];
        }
    }
    return [NSDictionary dictionaryWithDictionary:replaced];
}
@end
// you can use this category on your dictionaries.
NSDictionary *jsonDict = JSON;
jsondict = [jsonDict dictionaryByReplacingNullsWithStrings];