fix: getPeers size check (#2408)

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## Overview

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While trying to fix the testapp tutorial from the docs I stumbled upon
that issue.
By checking `[len(seeds) == 0]` before splitting, you avoid the common
pitfall where [`strings.Split("", ",")]` returns a slice of length 1
containing an empty string `[""]`.

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## Summary by CodeRabbit

* **Bug Fixes**
* Improved handling of empty input when processing peer seeds, ensuring
more efficient and accurate results.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: pthmas

pkg/sync/sync_service.go

@@ -382,10 +382,11 @@ func (syncService *SyncService[H]) getPeerIDs() []peer.ID {
 
 func getPeers(seeds string, logger log.Logger) []peer.ID {
 	var peerIDs []peer.ID
-	sl := strings.Split(seeds, ",")
-	if len(sl) == 0 {
+	if seeds == "" {
 		return peerIDs
 	}
+	sl := strings.Split(seeds, ",")
+
 	for _, seed := range sl {
 		maddr, err := multiaddr.NewMultiaddr(seed)
 		if err != nil {