fix: avoid panic when fork schedule is empty (#16175)

SortedForkSchedule should never be empty for a properly initialized
network schedule, but the handler already had a branch to support an
empty result. Without an early return, we wrote a JSON response and then
still accessed schedule[0], which could panic and double-write the HTTP
response in misconfigured setups.

---------

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

beacon-chain/rpc/eth/config/handlers.go

@@ -40,6 +40,7 @@ func GetForkSchedule(w http.ResponseWriter, r *http.Request) {
 		httputil.WriteJson(w, &structs.GetForkScheduleResponse{
 			Data: data,
 		})
+		return
 	}
 	previous := schedule[0]
 	for _, entry := range schedule {

changelog/sashass1315_fix-panic.md

@@ -0,0 +1,3 @@
+### Fixed
+
+- avoid panic when fork schedule is empty [#16175](https://github.com/OffchainLabs/prysm/pull/16175)