[SSE-C]: avoid encrypting empty objects. (#5525)

This change adds an object size check such that the server does not
encrypt empty objects (typically folders) for SSE-C. The server still
returns SSE-C headers but the object is not encrypted since there is no
point to encrypt such objects.

Fixes #5493

cmd/object-handlers.go

@@ -556,7 +556,7 @@ func (api objectAPIHandlers) PutObjectHandler(w http.ResponseWriter, r *http.Req
 		return
 	}
 	if objectAPI.IsEncryptionSupported() {
-		if IsSSECustomerRequest(r.Header) { // handle SSE-C requests
+		if IsSSECustomerRequest(r.Header) && size > 0 { // handle SSE-C requests
 			reader, err = EncryptRequest(hashReader, r, metadata)
 			if err != nil {
 				writeErrorResponse(w, toAPIErrorCode(err), r.URL)