Fix msearch request parsing when index expression is null (elastic#130776)

* Fix parsing when index expression is null

Author: smalyshev

docs/changelog/130776.yaml

@@ -0,0 +1,6 @@
+pr: 130776
+summary: Fix msearch request parsing when index expression is null
+area: Search
+type: bug
+issues:
+ - 129631

server/src/main/java/org/elasticsearch/common/xcontent/support/XContentMapValues.java

@@ -567,6 +567,9 @@ public static Map<String, Object> nodeMapValue(Object node, String desc) {
      * Otherwise the node is treated as a comma-separated string.
      */
     public static String[] nodeStringArrayValue(Object node) {
+        if (node == null) {
+            throw new ElasticsearchParseException("Expected a list of strings but got null");
+        }
         if (isArray(node)) {
             List<?> list = (List<?>) node;
             String[] arr = new String[list.size()];

server/src/test/java/org/elasticsearch/action/search/MultiSearchRequestTests.java

@@ -9,6 +9,7 @@
 
 package org.elasticsearch.action.search;
 
+import org.elasticsearch.ElasticsearchParseException;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.common.CheckedBiConsumer;
 import org.elasticsearch.common.Strings;
@@ -594,6 +595,14 @@ public void testFailOnExtraCharacters() throws IOException {
         }
     }
 
+    public void testNullIndex() throws IOException {
+        ElasticsearchParseException e = expectThrows(ElasticsearchParseException.class, () -> parseMultiSearchRequestFromString("""
+            {"index": null}
+            { "query": {"match_all": {}}}
+            """));
+        assertThat(e.getMessage(), containsString("Expected a list of strings but got null"));
+    }
+
     private static MultiSearchRequest mutate(MultiSearchRequest searchRequest) throws IOException {
         MultiSearchRequest mutation = copyRequest(searchRequest);
         List<CheckedRunnable<IOException>> mutators = new ArrayList<>();