Block-builder-scheduler: avoid committing offsets for partitions that have had no consumption

[seizethedave]

What this PR does

There was a bug introduced in #11867 where if a block-builder-scheduler encounters a partition in this state:

• the partition has an end offset > 0
• the block-builder consumer group has no commit for that partition

then:

• the block-builder-scheduler would store a committed offset of 0 in its state

which leads to:

1. the block-builder-scheduler would erroneously detect an offset gap the next time a job is created for that partition.
2. the scheduler would emit a value of 0 for the partition in the cortex_blockbuilder_scheduler_partition_committed_offset gauge.

(This can happen if the partition was hasn't been written to since before the scheduler or its consumer group existed in that environment.)

To fix this, I am introducing a special state of offsetEmpty (-1) to the offsets that are handled specially.

Checklist

• Tests updated.
• (n/a) Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• (n/a) about-versioning.md updated with experimental features.