gluon-config-mode-core: make welcome and reboot message optional

Make use of our new _translate() function.

package/gluon-config-mode-core/files/lib/gluon/web/view/gluon/config-mode/welcome.html

 <%-
 	local sysconfig = require 'gluon.sysconfig'
+
+	local msg = _translate('gluon-config-mode:welcome')
+	if not msg then return end
 -%>
 <p>
-	<%=
-		renderer.render_string(translate('gluon-config-mode:welcome'), {
+	<%
+		renderer.render_string(msg, {
 			hostname = hostname,
 			sysconfig = sysconfig,
 		})

package/gluon-config-mode-core/luasrc/lib/gluon/config-mode/reboot/0900-msg-reboot.lua

@@ -7,7 +7,8 @@ local uci = require("simple-uci").cursor()
 local hostname = pretty_hostname.get(uci)
 local contact = uci:get_first('gluon-node-info', 'owner', 'contact')
 
-local msg = translate('gluon-config-mode:reboot')
+local msg = _translate('gluon-config-mode:reboot')
+if not msg then return end
 
 renderer.render_string(msg, {
 	hostname = hostname,