Remove key length assertion LRUHandle::CalcTotalCharge (#6115)

Summary:
Inserting an entry in the block cache with 0 length key is a valid use case. Remove the assertion in ```LRUHandle::CalcTotalCharge```.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6115

Differential Revision: D18769693

Pulled By: anand1976

fbshipit-source-id: 34cc159650300dda6d7273480640478f28392cda

cache/lru_cache.h

@@ -133,7 +133,6 @@ struct LRUHandle {
   // Caclculate the memory usage by metadata
   inline size_t CalcTotalCharge(
       CacheMetadataChargePolicy metadata_charge_policy) {
-    assert(key_length);
     size_t meta_charge = 0;
     if (metadata_charge_policy == kFullChargeCacheMetadata) {
 #ifdef ROCKSDB_MALLOC_USABLE_SIZE