fix non-existent shard handling
fix non-existent shard handling

This commit removes `nil` shards returned from `tsdb.Store.Shards()`
which caused panics in some SELECTs. This can occur if the meta
store has created shards before the store or if the shards are
distributed throughout a cluster.

Fixes #5555

jonseymour/influxdb,li-ang/influxdb,nooproblem/influxdb,chenziliang/influxdb,mark-rushakoff/influxdb,daachi/influxdb,mnuessler/influxdb,pkittenis/influxdb,arussellsaw/influxdb,arussellsaw/influxdb,runner-mei/influxdb,jipperinbham/influxdb,jonseymour/influxdb,nooproblem/influxdb,otoolep/influxdb,archsaber/influxdb,joaopedrotaveira/influxdb,joaopedrotaveira/influxdb,archsaber/influxdb,jonseymour/influxdb,mnuessler/influxdb,influxdata/influxdb,jonseymour/influxdb,daachi/influxdb,vladlopes/influxdb,benbjohnson/influxdb,chenziliang/influxdb,archsaber/influxdb,pkittenis/influxdb,vladlopes/influxdb,nooproblem/influxdb,runner-mei/influxdb,joaopedrotaveira/influxdb,apigee/influxdb,mark-rushakoff/influxdb,chenziliang/influxdb,linearb/influxdb,jipperinbham/influxdb,mark-rushakoff/influxdb,pkittenis/influxdb,linearb/influxdb,jgeiger/influxdb,nooproblem/influxdb,mnuessler/influxdb,daachi/influxdb,benbjohnson/influxdb,arussellsaw/influxdb,pkittenis/influxdb,otoolep/influxdb,daachi/influxdb,influxdb/influxdb,vladlopes/influxdb,linearb/influxdb,influxdb/influxdb,jgeiger/influxdb,archsaber/influxdb,benbjohnson/influxdb,pkittenis/influxdb,pkittenis/influxdb,influxdb/influxdb,benbjohnson/influxdb,jonseymour/influxdb,influxdata/influxdb,otoolep/influxdb,jipperinbham/influxdb,arussellsaw/influxdb,mark-rushakoff/influxdb,jipperinbham/influxdb,otoolep/influxdb,daachi/influxdb,jgeiger/influxdb,arussellsaw/influxdb,joaopedrotaveira/influxdb,mnuessler/influxdb,jipperinbham/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,influxdb/influxdb,arussellsaw/influxdb,joaopedrotaveira/influxdb,nooproblem/influxdb,mvadu/influxdb,apigee/influxdb,daachi/influxdb,apigee/influxdb,mnuessler/influxdb,runner-mei/influxdb,apigee/influxdb,influxdb/influxdb,runner-mei/influxdb,mvadu/influxdb,jgeiger/influxdb,mnuessler/influxdb,jonseymour/influxdb,mvadu/influxdb,linearb/influxdb,mvadu/influxdb,mvadu/influxdb,jipperinbham/influxdb,apigee/influxdb,chenziliang/influxdb,apigee/influxdb,otoolep/influxdb,jgeiger/influxdb,vladlopes/influxdb,otoolep/influxdb,li-ang/influxdb,jgeiger/influxdb,mark-rushakoff/influxdb,linearb/influxdb,mvadu/influxdb