eth/protocols/eth: handle zero-count header requests (ethereum#30305)

Proper fix for handling `count=0` get header requests. 

https://en.wikipedia.org/wiki/Count_Zero

eth/protocols/eth/handlers.go

@@ -42,6 +42,9 @@ func handleGetBlockHeaders(backend Backend, msg Decoder, peer *Peer) error {
 // ServiceGetBlockHeadersQuery assembles the response to a header query. It is
 // exposed to allow external packages to test protocol behavior.
 func ServiceGetBlockHeadersQuery(chain *core.BlockChain, query *GetBlockHeadersRequest, peer *Peer) []rlp.RawValue {
+	if query.Amount == 0 {
+		return nil
+	}
 	if query.Skip == 0 {
 		// The fast path: when the request is for a contiguous segment of headers.
 		return serviceContiguousBlockHeaderQuery(chain, query)