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

@@ -42,6 +42,9 @@ Bug Fixes
 * SOLR-17098: ZK Credentials and ACLs are no longer sent to all ZK Servers when using Streaming Expressions.
   They will only be used when sent to the default ZK Host. (Houston Putman, Jan Høydahl, David Smiley, Gus Heck, Qing Xu)
 
+* 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)
+
 Optimizations
 ---------------------
 * SOLR-16555: SolrIndexSearcher - FilterCache intersections/andNot should not clone bitsets repeatedly (Kevin Risden, David Smiley)

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

@@ -959,8 +959,13 @@ private void applyOlderUpdates(@SuppressWarnings({"rawtypes"})SolrDocumentBase n
     for (String fieldName : olderDoc.getFieldNames()) {
       // 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);
+          }
         }
       }
     }