fix(mocks): stop the mock advertising invalid licence keys

mock-storage/licence.json held the placeholder word 'invalid' for both keys, so
startupservice returned them as globvars and every HTTPS site showed the
'Licence key is invalid / Error decrypting license key' page. An empty pair is
what an unlicensed install has, and it lands on the free tier instead.

sas/mocks/sasjs/mock-storage/licence.json

@@ -1,4 +1,4 @@
 {
-  "licenceKey": "invalid",
-  "activationKey": "invalid"
-}
+  "licenceKey": "",
+  "activationKey": ""
+}