Make plan_cluster_use_sort cope with no IndexOptInfo for the target index.

The original coding assumed that such a case represents caller error, but
actually get_relation_info will omit generating an IndexOptInfo for any
index it thinks is unsafe to use.  Therefore, handle this case by returning
"true" to indicate that a seqscan-and-sort is the preferred way to
implement the CLUSTER operation.  New bug in 9.1, no backpatch needed.
Per bug #5985 from Daniel Grace.