[compiler] fix pksorted case of LoweredTableReader

[patrick-schultz]

Fixes #11335.

@tpoterba made the replicating test, with that it was easy to find the source of the bug.

The error was in

pkPartitioned
  .strictify()
  ...
  .changePartitionerNoRepartition(partitioner.extendKeySamePartitions(keyType))

where pkPartitioned is keyed by the partition key. In the test case, all rows have the same partition key, so the partitioner looks like [x, x], [x, x], .... In that case, strictify correctly collapses all those partitions into one, but partitioner.extendKeySamePartitions(keyType) tries to extend the key type without changing the partitioning, which in this case creates an invalid partitioner.

The fix is to use pkPartitioned.extendKeyPreservesPartitioning(key), which does the strictify and creates the correct partitioner.