Fix panic in Redis key metricset (elastic#13426) (elastic#13435)

If a key is removed during a fetch, `FetchKeyInfo` returns a nil object,
this nil object should be ignored, if passed to `eventMapping` it
panics.

(cherry picked from commit 318a041)

CHANGELOG.next.asciidoc

@@ -96,6 +96,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix `logstash/node_stats` metricset to also collect `logstash_stats.events.duration_in_millis` field when `xpack.enabled: true` is set. {pull}13082[13082]
 - Fix `logstash/node` metricset to also collect `logstash_state.pipeline.representation.{type,version,hash}` fields when `xpack.enabled: true` is set. {pull}13133[13133]
 - Check if fields in DBInstance is nil in rds metricset. {pull}13294[13294] {issue}13037[13037]
+- Fix panic in Redis Key metricset when collecting information from a removed key. {pull}13426[13426]
 
 *Packetbeat*
 

metricbeat/module/redis/key/key.go

@@ -112,6 +112,10 @@ func (m *MetricSet) Fetch(r mb.ReporterV2) error {
 				r.Error(err)
 				continue
 			}
+			if keyInfo == nil {
+				m.Logger().Debugf("Ignoring removed key %s from keyspace %d", key, keyspace)
+				continue
+			}
 			event := eventMapping(keyspace, keyInfo)
 			if !r.Event(event) {
 				m.Logger().Debug("Failed to report event, interrupting fetch")