protocol: allow to send a zero output refresh rate

Fixed refresh rate doesn't always make sense for all outputs. In case
it's not available or not relevant, allow compositors to send zero.

For instance the can be the case for virtual outputs.

Signed-off-by: Simon Ser <contact@emersion.fr>
1 job for zero-refresh in 1 minute and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #266330
build-native

00:01:34