fix: incorrect nil logic for segments

v3/go.mod

@@ -7,7 +7,6 @@ require (
 	google.golang.org/protobuf v1.34.2
 )
 
-
 retract v3.22.0 // release process error corrected in v3.22.1
 
 retract v3.25.0 // release process error corrected in v3.25.1

v3/newrelic/transaction.go

@@ -333,7 +333,11 @@ func (txn *Transaction) startSegmentAt(at time.Time) SegmentStartTime {
 //	// ... code you want to time here ...
 //	segment.End()
 func (txn *Transaction) StartSegment(name string) *Segment {
-	if IsSecurityAgentPresent() && nilTransaction(txn) && txn.thread.thread.threadID > 0 {
+	if nilTransaction(txn) {
+		return &Segment{} // return a non-nil Segment to avoid nil dereference
+	}
+
+	if IsSecurityAgentPresent() && txn.thread.thread != nil && txn.thread.thread.threadID > 0 {
 		// async segment start
 		secureAgent.SendEvent("NEW_GOROUTINE_LINKER", txn.thread.getCsecData())
 	}