Restrict client metadata update more

If Metadata.Full is set to false, and no topics have ever been
requested, the first background refresh would load them all because go's
varags don't distinguish between nothing and an explicitly splatted
empty array.

Just break instead.

client.go

@@ -612,6 +612,9 @@ func (client *client) backgroundMetadataUpdater() {
 				if specificTopics, err := client.Topics(); err != nil {
 					Logger.Println("Client background metadata topic load:", err)
 					break
+				} else if len(specificTopics) == 0 {
+					Logger.Println("Client background metadata update: no specific topics to update")
+					break
 				} else {
 					topics = specificTopics
 				}