Small cleanup of create_list_bounds()

When checking for interleaved partitions, we mark the partition as
interleaved when;

1. we find an earlier partition index when looping over the
sorted-by-Datum indexes[] array, or;

2. we find that the NULL partition allows some non-NULL Datum value.

In the code, as it was written in db632fbca we'll continue to check for
case 2 when we've already marked the partition as interleaved for case 1.
Here we make it so we don't bother marking the partition as interleaved
for case 2 when it's already been marked due to case 1.

Really all this saves is a useless call to bms_add_member(), but since
this code is new to PG15, it seems worth fixing it now to save anyone the
trouble of complaining at some time in the future.  We have the
opportunity to improve this now before PG15 is out.  This might ease some
future back-patching pain.

Per report and patch by Zhihong Yu.  However, I slightly revised the
comments and altered the bms_add_member() code to match in both locations.
We already know that index is equal to boundinfo->null_index from the if
condition.

Author: Zhihong Yu
Discussion: https://postgr.es/m/CALNJ-vQbZR0pYxz9zQ5bqXVcwtGgNgVupeEpNT65HZ+yWZnc4g@mail.gmail.com
Backpatch-through: 15, same as db632fbca.

src/backend/partitioning/partbounds.c

@@ -648,13 +648,14 @@ create_list_bounds(PartitionBoundSpec **boundspecs, int nparts,
 																  index);
 
 				/*
-				 * Mark the NULL partition as interleaved if we find that it
-				 * allows some other non-NULL Datum.
+				 * Otherwise, if the null_index exists in the indexes array,
+				 * then the NULL partition must also allow some other Datum,
+				 * therefore it's "interleaved".
 				 */
-				if (partition_bound_accepts_nulls(boundinfo) &&
-					index == boundinfo->null_index)
+				else if (partition_bound_accepts_nulls(boundinfo) &&
+						 index == boundinfo->null_index)
 					boundinfo->interleaved_parts = bms_add_member(boundinfo->interleaved_parts,
-																  boundinfo->null_index);
+																  index);
 
 				last_index = index;
 			}