Merge pull request #2389 from zhongwuzw/fix-nullable-5.x

Fix nullable key when get image load operation

SDWebImage/UIView+WebCacheOperation.m

@@ -31,10 +31,12 @@ typedef NSMapTable<NSString *, id<SDWebImageOperation>> SDOperationsDictionary;
 
 - (nullable id<SDWebImageOperation>)sd_imageLoadOperationForKey:(nullable NSString *)key  {
     id<SDWebImageOperation> operation;
+    if (key) {
         SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
         @synchronized (self) {
             operation = [operationDictionary objectForKey:key];
         }
+    }
     return operation;
 }