Workaround: make sure we don't send completely empty map to NM back when asking for VPN secrets

When NM asks for secrets, which should be system-owned (stored in NM), it also asks our
secret agent from some reason if we have them, but if we send back an empty map, it won't ask
again with required flag which would invoke displaying an auth dialog. We have to send back a
map containing "secrets" key which should be without any value. It worked before that way
because in NetworkManagerQt we always returned this map with secrets even when it was empty.

BUG:339296
CCBUG:309931
CCBUG:334474