filer: restore fallback to chunkCache when cacher returns no data

Fix critical issue where ReadChunkAt would return 0,nil immediately
if SingleChunkCacher couldn't provide data for the requested offset,
without trying the chunkCache fallback. Now if cacher.readChunkAt
returns n=0 and err=nil, we fall through to try chunkCache.

weed/filer/reader_cache.go

@@ -99,7 +99,13 @@ func (rc *ReaderCache) ReadChunkAt(ctx context.Context, buffer []byte, fileId st
 
 	if cacher, found := rc.downloaders[fileId]; found {
 		rc.Unlock()
-		return cacher.readChunkAt(ctx, buffer, offset)
+		n, err := cacher.readChunkAt(ctx, buffer, offset)
+		if n > 0 || err != nil {
+			return n, err
+		}
+		// If n=0 and err=nil, the cacher couldn't provide data for this offset.
+		// Fall through to try chunkCache.
+		rc.Lock()
 	}
 	if shouldCache || rc.lookupFileIdFn == nil {
 		n, err := rc.chunkCache.ReadChunkAt(buffer, fileId, uint64(offset))