Fix append of possible nil iterator.
Fix append of possible nil iterator.

This commit updates an iterator list to ignore `nil` iterators.
Adding a `nil` caused the `SeriesIterators.Close()` to panic.

influxdb/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,influxdata/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdata/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,li-ang/influxdb,li-ang/influxdb,influxdb/influxdb,influxdb/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb