Fix user ssh key exporting and tests (#37256) (#37258)

Backport #37256 by wxiaoguang

1. Make sure OmitEmail won't panic
2. SSH principal keys are not for signing or authentication

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/user/home.go

@@ -655,6 +655,9 @@ func ShowSSHKeys(ctx *context.Context) {
 	// "authorized_keys" file format: "#" followed by comment line per key
 	buf.WriteString("# Gitea isn't a key server. The keys are exported as the user uploaded and might not have been fully verified.\n")
 	for i := range keys {
+		if keys[i].Type == asymkey_model.KeyTypePrincipal {
+			continue // SSH principal keys are not for signing or authentication
+		}
 		buf.WriteString(keys[i].OmitEmail())
 		buf.WriteString("\n")
 	}