Deduplicate min and max term in single-term FieldReader (#13618)

I noticed that single-term readers are an edge case but not that
uncommon in Elasticsearch heap dumps. It seems quite common to have a
constant value for some field across a complete segment (e.g. a version
value that is repeated endlessly in logs).
Seems simple enough to deduplicate here to save a couple MB of heap.

lucene/core/src/java/org/apache/lucene/codecs/lucene90/blocktree/Lucene90BlockTreeTermsReader.java

@@ -200,6 +200,11 @@ public Lucene90BlockTreeTermsReader(PostingsReaderBase postingsReader, SegmentRe
             final int docCount = metaIn.readVInt();
             BytesRef minTerm = readBytesRef(metaIn);
             BytesRef maxTerm = readBytesRef(metaIn);
+            if (numTerms == 1) {
+              assert maxTerm.equals(minTerm);
+              // save heap for edge case of a single term only so min == max
+              maxTerm = minTerm;
+            }
             if (docCount < 0
                 || docCount > state.segmentInfo.maxDoc()) { // #docs with field must be <= #docs
               throw new CorruptIndexException(

lucene/core/src/java/org/apache/lucene/codecs/lucene90/blocktree/Lucene90BlockTreeTermsWriter.java

@@ -598,8 +598,6 @@ private void append(
   private final ByteBuffersDataOutput scratchBytes = ByteBuffersDataOutput.newResettableInstance();
   private final IntsRefBuilder scratchIntsRef = new IntsRefBuilder();
 
-  static final BytesRef EMPTY_BYTES_REF = new BytesRef();
-
   private static class StatsWriter {
 
     private final DataOutput out;