nip22: prevent panic, return nil if not found according to nip10.go

nip22/nip22.go

@@ -4,21 +4,24 @@ import "github.com/nbd-wtf/go-nostr"
 
 func GetThreadRoot(tags nostr.Tags) *nostr.Tag {
 	for _, tag := range tags {
+		if len(tag) < 2 {
+			continue
+		}
 		if tag[0] == "E" || tag[0] == "A" || tag[0] == "I" {
 			return &tag
 		}
 	}
-	empty := nostr.Tag{}
-	return &empty
+	return nil
 }
 
 func GetImmediateReply(tags nostr.Tags) *nostr.Tag {
 	for _, tag := range tags {
+		if len(tag) < 2 {
+			continue
+		}
 		if tag[0] == "e" || tag[0] == "a" || tag[0] == "i" {
 			return &tag
 		}
 	}
-
-	empty := nostr.Tag{}
-	return &empty
+	return nil
 }