Fix daemon exception if config key isn't present

[sqozz]

Currently linphone-daemon crashes if you try to read a config key/value which isn't present because NULL is passed to the ConfigResponse constructor:

selenium usr/bin » ./linphone-daemon --pipe lpdaemon --config test-config & sleep 1 && (echo "config-get baz bar" | nc -U /tmp/lpdaemon)
[1] 10721
Server unix socket created, name=lpdaemon fd=3
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid
[1]  + 10721 abort      ./linphone-daemon --pipe lpdaemon --config test-config

With my change applied, the command behaves like documented:

selenium usr/bin » ./linphone-daemon --pipe lpdaemon --config test-config & sleep 1 && (echo "config-get baz bar" | nc -U /tmp/lpdaemon)
[1] 4366
Server unix socket created, name=lpdaemon fd=3
Status: Ok

Value: <unset>