🐛 zb: Make PropertyStream yield changes if service starts later

This seems like the simplest way to address
#1111.

Before this, a `PropertyStream` created when a service wasn't running
would never yield a value, even if the service started later and send
property change signals.

But if a service stopped, and then was restarted, property changes would
be received.

It seems `PropertiesCache::init()` was returning an error, from trying
to call `GetAll` with a destination that doesn't exist.

Instead, let `init()` complete successfully (without populating the
cache), so it returns a `PropertiesChangedStream` which is then
monitored for changes.

It may also be good if it yielded values when a service takes ownership
of the name, but that's a separate issue since it also applies to
service restarts. And an application can handle `OwnerChanged` itself
relatively easily.

Author: ids1024

zbus/src/proxy/mod.rs

@@ -355,6 +355,14 @@ impl PropertiesCache {
                 Some(Either::Left(_update)) => {
                     // discard updates prior to the initial population
                 }
+                // FIXME: Use deref pattern when stable
+                // https://github.com/rust-lang/rust/issues/87121
+                Some(Either::Right(Err(Error::FDO(fdo))))
+                    if matches!(*fdo, fdo::Error::ServiceUnknown(_)) =>
+                {
+                    // DBus service doesn't exist. But may appear later on the bus.
+                    break;
+                }
                 Some(Either::Right(populate)) => {
                     populate?.body().deserialize().map(|values| {
                         self.update_cache(&uncached_properties, &values, &[], &interface);