SOLR-11598: Fix bug while setting and resetting string doc-values while exporting documents

solr/core/src/java/org/apache/solr/handler/export/StringValue.java

@@ -96,10 +96,10 @@ class StringValue implements SortValue {
   }
 
   public void setNextReader(LeafReaderContext context) throws IOException {
-    if (globalDocValues instanceof MultiDocValues.MultiSortedDocValues) {
+    if (ordinalMap != null) {
       toGlobal = ordinalMap.getGlobalOrds(context.ord);
-      docValues = DocValues.getSorted(context.reader(), field);
     }
+    docValues = DocValues.getSorted(context.reader(), field);
     lastDocID = 0;
   }
 

solr/core/src/test/org/apache/solr/handler/export/TestExportWriter.java

@@ -84,6 +84,29 @@ public class TestExportWriter extends SolrTestCaseJ4 {
 
   }
 
+  @Test
+  public void testSortingOnFieldWithNoValues() throws Exception {
+    assertU(delQ("*:*"));
+    assertU(commit());
+
+    assertU(adoc("id","1"));
+    assertU(commit());
+
+    // 10 fields
+    List<String> fieldNames = new ArrayList<>(Arrays.asList("floatdv", "intdv", "stringdv", "longdv", "doubledv",
+        "datedv", "booleandv", "field1_s_dv", "field2_i_p", "field3_l_p"));
+    for (String sortField : fieldNames) {
+      String resp = h.query(req("q", "*:*", "qt", "/export", "fl", "id," + sortField, "sort", sortField + " desc"));
+      assertJsonEquals(resp, "{\n" +
+          "  \"responseHeader\":{\"status\":0},\n" +
+          "  \"response\":{\n" +
+          "    \"numFound\":1,\n" +
+          "    \"docs\":[{\n" +
+          "        \"id\":\"1\"}]}}");
+    }
+
+  }
+
   public static void createIndex() {
     assertU(adoc("id","1",
                  "floatdv","2.1",