core/rpc: fix for null entries in log filters. Closes #725

You can now specify `null` as a way of saying "not interested in this
topic, match all". core.Filter assumes the zero'd address to be the
wildcard. JSON rpc assumes empty strings to be wildcards.

core/filter.go

@@ -134,7 +134,8 @@ Logs:
 		for i, topics := range self.topics {
 			for _, topic := range topics {
 				var match bool
-				if log.Topics[i] == topic {
+				// common.Hash{} is a match all (wildcard)
+				if (topic == common.Hash{}) || log.Topics[i] == topic {
 					match = true
 				}
 				if !match {