Support empty first line in msearch request body (#41011)

With #41442 we have deprecated support for empty line before any action metadata in msearch API. With this commit we remove support for such empty line, in place of parsing it as empty action metadata, which was previously not supported although the following items could have an empty line representing their corresponding action metadata. This way we make all times equal.

Relates to #39841

server/src/main/java/org/elasticsearch/action/search/MultiSearchRequest.java

@@ -183,12 +183,6 @@ public class MultiSearchRequest extends ActionRequest implements CompositeIndice
             if (nextMarker == -1) {
                 break;
             }
-            // support first line with \n
-            if (nextMarker == 0) {
-                from = nextMarker + 1;
-                continue;
-            }
-
             SearchRequest searchRequest = new SearchRequest();
             if (indices != null) {
                 searchRequest.indices(indices);

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

@@ -48,6 +48,7 @@ import static java.util.Collections.singletonList;
 import static org.elasticsearch.search.RandomSearchRequestGenerator.randomSearchRequest;
 import static org.elasticsearch.test.EqualsHashCodeTestUtils.checkEqualsAndHashCode;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.nullValue;
 
@@ -180,6 +181,15 @@ public class MultiSearchRequestTests extends ESTestCase {
         assertThat(request.requests().get(2).routing(), equalTo("123"));
     }
 
+    public void testNoMetadata() throws Exception {
+        MultiSearchRequest request = parseMultiSearchRequest("/org/elasticsearch/action/search/msearch-no-metadata.json");
+        assertThat(request.requests().size(), equalTo(4));
+        for (SearchRequest searchRequest : request.requests()) {
+            assertThat(searchRequest.indices().length, equalTo(0));
+            assertThat(searchRequest.source().query(), instanceOf(MatchAllQueryBuilder.class));
+        }
+    }
+
     public void testResponseErrorToXContent() {
         long tookInMillis = randomIntBetween(1, 1000);
         MultiSearchResponse response = new MultiSearchResponse(

server/src/test/resources/org/elasticsearch/action/search/msearch-no-metadata.json

@@ -0,0 +1,8 @@
+
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}