SOLR-17120: handle null value when merging partials (#2214)

* SOLR-17120 handle null value when merging partials

  - this change avoids a `NullPointerException` that can occur
    under some circumstances when performing multiple partial
    updates of the same document

solr/CHANGES.txt

@@ -195,6 +195,9 @@ Bug Fixes
 
 * SOLR-17074: Fixed not correctly escaped quote in bin/solr script (Dominique Béjean, Vincenzo D'Amore)
 
+* SOLR-17120: Fix NullPointerException in UpdateLog.applyOlderUpdates that can occur if there are multiple partial
+  updates of the same document in separate requests using commitWithin. (Calvin Smith, Christine Poerschke)
+
 Dependency Upgrades
 ---------------------
 * SOLR-17012: Update Apache Hadoop to 3.3.6 and Apache Curator to 5.5.0 (Kevin Risden)

solr/core/src/java/org/apache/solr/update/UpdateLog.java

@@ -1020,8 +1020,13 @@ private void applyOlderUpdates(
       // if the newerDoc has this field, then this field from olderDoc can be ignored
       if (!newerDoc.containsKey(fieldName)
           && (mergeFields == null || mergeFields.contains(fieldName))) {
-        for (Object val : olderDoc.getFieldValues(fieldName)) {
-          newerDoc.addField(fieldName, val);
+        Collection<Object> values = olderDoc.getFieldValues(fieldName);
+        if (values == null) {
+          newerDoc.addField(fieldName, null);
+        } else {
+          for (Object val : values) {
+            newerDoc.addField(fieldName, val);
+          }
         }
       }
     }