Get rid of log messages from SrvKeyspaceWatcher when no node/key

Signed-off-by: Matt Lord <mattalord@gmail.com>

go/vt/srvtopo/watch.go

@@ -195,7 +195,7 @@ func (entry *watchEntry) onErrorLocked(err error, init bool) {
 		// This watcher will able to continue to return the last value till it is not able to connect to the topo server even if the cache TTL is reached.
 		// TTL cache is only checked if the error is a known error i.e topo.Error.
 		_, isTopoErr := err.(topo.Error)
-		if isTopoErr && time.Since(entry.lastValueTime) > entry.rw.cacheTTL {
+		if entry.value != nil && isTopoErr && time.Since(entry.lastValueTime) > entry.rw.cacheTTL {
 			log.Errorf("WatchSrvKeyspace clearing cached entry for %v", entry.key)
 			entry.value = nil
 		}