Avoid allocating liveDocs for no soft-deletes (apache#13895)

This is a continuation of apache#13588, where we avoided allocating liveDocs 
for segments that have the __soft_deletes field but no values in it.
However, that PR only addressed the reading side. This change fixes the
writing scenario with IndexWriter.

Relates apache#13588

lucene/core/src/java/org/apache/lucene/index/PendingSoftDeletes.java

@@ -76,15 +76,14 @@ void onNewReader(CodecReader reader, SegmentCommitInfo info) throws IOException
     hardDeletes.onNewReader(reader, info);
     // only re-calculate this if we haven't seen this generation
     if (dvGeneration < info.getDocValuesGen()) {
-      final DocIdSetIterator iterator =
-          FieldExistsQuery.getDocValuesDocIdSetIterator(field, reader);
-      int newDelCount;
-      if (iterator
-          != null) { // nothing is deleted we don't have a soft deletes field in this segment
-        assert info.info.maxDoc() > 0 : "maxDoc is 0";
+      final int newDelCount;
+      var iterator = FieldExistsQuery.getDocValuesDocIdSetIterator(field, reader);
+      if (iterator != null && iterator.nextDoc() != DocIdSetIterator.NO_MORE_DOCS) {
+        iterator = FieldExistsQuery.getDocValuesDocIdSetIterator(field, reader);
         newDelCount = applySoftDeletes(iterator, getMutableBits());
         assert newDelCount >= 0 : " illegal pending delete count: " + newDelCount;
       } else {
+        // nothing is deleted we don't have a soft deletes field in this segment
         newDelCount = 0;
       }
       assert info.getSoftDelCount() == newDelCount

lucene/core/src/test/org/apache/lucene/index/TestSoftDeletesDirectoryReaderWrapper.java

@@ -306,6 +306,12 @@ public void testAvoidWrappingReadersWithoutSoftDeletes() throws Exception {
                   softDeletesField, MatchNoDocsQuery::new, mergePolicy));
       writer.forceMerge(1);
       try (DirectoryReader reader = DirectoryReader.open(writer)) {
+        for (LeafReaderContext leafContext : reader.leaves()) {
+          assertThat(leafContext.reader(), instanceOf(SegmentReader.class));
+          SegmentReader segmentReader = (SegmentReader) leafContext.reader();
+          assertNull(segmentReader.getLiveDocs());
+          assertNull(segmentReader.getHardLiveDocs());
+        }
         SoftDeletesDirectoryReaderWrapper wrapped =
             new SoftDeletesDirectoryReaderWrapper(reader, softDeletesField);
         assertEquals(numDocs, wrapped.numDocs());