xds: allow empty delta update

[kkHAIKE]

Commit Message:
when delta xds recv empty repsonce, it should be call onConfigUpdate, instead of do nothing, and stop init process 15s (initial_fetch_timeout)

Additional Description:
a first delta xds repsonce can't call wildcard subscription's onConfigUpdate (ex: empty CDS), and stop init process 15s. It don't like SOTW part:

envoy/source/common/config/watch_map.cc

Lines 104 to 115 in 7f4e35e

	if (this_watch_updates == per_watch_updates.end()) {
	// This update included no resources this watch cares about.
	// 1) If there is only a single, wildcard watch (i.e. Cluster or Listener), always call
	// its onConfigUpdate even if just a no-op, to properly maintain state-of-the-world
	// semantics and the update_empty stat.
	// 2) If this watch previously had some resources, it means this update is removing all
	// of this watch's resources, so the watch must be informed with an onConfigUpdate.
	// 3) Otherwise, we can skip onConfigUpdate for this watch.
	if (map_is_single_wildcard || !watch->state_of_the_world_empty_) {
	watch->state_of_the_world_empty_ = true;
	watch->callbacks_.onConfigUpdate({}, version_info);
	}

Risk Level: Low
Testing: Manual Pass
Docs Changes: None
Release Notes: None

[kkHAIKE]

ping @htuch 😄

[htuch]

Looks good; if you really want to future proof this you could also add an integration test, but don't feel super strong on that. One tiny nit.
/wait

[stale]

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[htuch]

LGTM, thanks!