Add note to GPG key response if user has no keys (#18961)

* Prevent invalid key response.

* Display note instead of 404 response.

* Fixed test.

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

routers/web/user/home.go

@@ -736,6 +736,7 @@ func ShowGPGKeys(ctx *context.Context, uid int64) {
 		ctx.ServerError("ListGPGKeys", err)
 		return
 	}
+
 	entities := make([]*openpgp.Entity, 0)
 	failedEntitiesID := make([]string, 0)
 	for _, k := range keys {
@@ -755,6 +756,8 @@ func ShowGPGKeys(ctx *context.Context, uid int64) {
 	headers := make(map[string]string)
 	if len(failedEntitiesID) > 0 { // If some key need re-import to be exported
 		headers["Note"] = fmt.Sprintf("The keys with the following IDs couldn't be exported and need to be reuploaded %s", strings.Join(failedEntitiesID, ", "))
+	} else if len(entities) == 0 {
+		headers["Note"] = "This user hasn't uploaded any GPG keys."
 	}
 	writer, _ := armor.Encode(&buf, "PGP PUBLIC KEY BLOCK", headers)
 	for _, e := range entities {