Fix the subset method on tags

If a point had no tags at all and was asked for the subset of tags with
at least one key, it would return a new set of tags that was completely
empty. In contrast, if the point had any tags at all, it would return a
set of tags with the tag value being an empty string. This lead to
a point with no tags being treated differently than a point with at
least one tag.

Fixing this so the tag value will always be an empty string for
consistency. A missing tag should always be empty.

CHANGELOG.md

@@ -12,6 +12,7 @@
 - [#6257](https://github.com/influxdata/influxdb/issues/6257): CreateShardGroup was incrementing meta data index even when it was idempotent.
 - [#6223](https://github.com/influxdata/influxdb/issues/6223): Failure to start/run on Windows. Thanks @mvadu
 - [#6229](https://github.com/influxdata/influxdb/issues/6229): Fixed aggregate queries with no GROUP BY to include the end time.
+- [#6283](https://github.com/influxdata/influxdb/pull/6283): Fix GROUP BY tag to produce consistent results when a series has no tags.
 
 ## v0.12.0 [2016-04-05]
 ### Release Notes

influxql/point.go

@@ -89,7 +89,7 @@ func (t *Tags) Value(k string) string {
 
 // Subset returns a new tags object with a subset of the keys.
 func (t *Tags) Subset(keys []string) Tags {
-	if t.m == nil || len(keys) == 0 {
+	if len(keys) == 0 {
 		return Tags{}
 	}