Improve ProviderCache

What are you think about add to ProviderCache flag isAllowEmptyResult?

For example:

$chain = new \Geocoder\Provider\Chain\Chain([
    // ...
]);
$cache = new \Geocoder\Provider\Cache\ProviderCache($chain, $cache, 86400);

When have exceptions from providers the chain catch and return empty ArrayCollection.
This result is saved for 24h. I think will be good point if I don't save empty results.

https://github.com/geocoder-php/Geocoder/blob/35650985d8b69ead88340ca5b75546f7117fad06/src/Provider/Cache/ProviderCache.php#L65-L66

Here is example how I think can be:

         $result = $this->realProvider->geocodeQuery($query); 
         if (!$result->isEmpty() || $this->isAllowEmptyResult)) {
             $this->cache->set($cacheKey, $result, $this->lifetime);
         }

What are you think about it?

从 src/Provider/Cache/ProviderCache.php 开始，重点查看第 65–66 行附近的缓存写入，并追踪 Chain 如何在 provider 异常后返回空的 ArrayCollection。定义建议的 isAllowEmptyResult 选项应如何影响缓存，然后验证空结果默认会被跳过，并且在该选项允许时会被存储。