WAL loading: don't send empty buffers over chan (#11319)
WAL loading: don't send empty buffers over chan (#11319)

If some shards did not get any samples mapped, the buffer will be empty
so sending it over the chan to `processWALSamples()` is a waste of time.

This is especially likely now we are checking `minValidTime` before
sending.

Signed-off-by: Bryan Boreham <de652539b24efdf404b95def4635bf3dbfd846aa@gmail.com>

Signed-off-by: Bryan Boreham <de652539b24efdf404b95def4635bf3dbfd846aa@gmail.com>
prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus