Fix fleet search API with no checkpoints

x-pack/plugin/fleet/qa/rest/src/yamlRestTest/resources/rest-api-spec/test/fleet/20_wait_for_checkpoints.yml

@@ -32,6 +32,28 @@ setup:
   - do:
       indices.refresh: {}
 
+---
+"Execute successful search with wait_for_checkpoints default":
+  - do:
+      fleet.search:
+        index: "test-after-refresh"
+        allow_partial_search_results: false
+
+        body: { query: { match_all: {} } }
+
+  - match: { _shards.successful: 1 }
+  - match: { hits.total.value: 2 }
+
+  - do:
+      fleet.search:
+        index: "test-after-refresh"
+        allow_partial_search_results: false
+        wait_for_checkpoints: []
+        body: { query: { match_all: { } } }
+
+  - match: { _shards.successful: 1 }
+  - match: { hits.total.value: 2 }
+
 ---
 "Execute successful after refresh search":
   - do:
@@ -114,3 +136,19 @@ setup:
   - match: { responses.1._shards.successful: 1 }
   - match: { responses.1.hits.total.value: 2 }
   - match: { responses.2.error.caused_by.type: "illegal_argument_exception" }
+
+---
+"Test msearch wait_for_checkpoints default":
+  - do:
+      fleet.msearch:
+        index: "test-after-refresh"
+        body:
+          - { "allow_partial_search_results": false }
+          - { query: { match_all: { } } }
+          - { "allow_partial_search_results": false, wait_for_checkpoints: [] }
+          - { query: { match_all: { } } }
+
+  - match: { responses.0._shards.successful: 1 }
+  - match: { responses.0.hits.total.value: 2 }
+  - match: { responses.1._shards.successful: 1 }
+  - match: { responses.1.hits.total.value: 2 }

x-pack/plugin/fleet/src/main/java/org/elasticsearch/xpack/fleet/rest/RestFleetMultiSearchAction.java

@@ -68,7 +68,9 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
                     for (int i = 0; i < stringWaitForCheckpoints.length; ++i) {
                         waitForCheckpoints[i] = Long.parseLong(stringWaitForCheckpoints[i]);
                     }
-                    searchRequest.setWaitForCheckpoints(Collections.singletonMap("*", waitForCheckpoints));
+                    if (waitForCheckpoints.length != 0) {
+                        searchRequest.setWaitForCheckpoints(Collections.singletonMap("*", waitForCheckpoints));
+                    }
                     return true;
                 } else if ("wait_for_checkpoints_timeout".equals(key)) {
                     final TimeValue waitForCheckpointsTimeout = nodeTimeValue(value, TimeValue.timeValueSeconds(30));
@@ -96,7 +98,9 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
                 }
             }
             long[] checkpoints = searchRequest.getWaitForCheckpoints().get("*");
-            searchRequest.setWaitForCheckpoints(Collections.singletonMap(indices[0], checkpoints));
+            if (checkpoints != null) {
+                searchRequest.setWaitForCheckpoints(Collections.singletonMap(indices[0], checkpoints));
+            }
         }
 
         return channel -> {

x-pack/plugin/fleet/src/main/java/org/elasticsearch/xpack/fleet/rest/RestFleetSearchAction.java

@@ -76,7 +76,9 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
                             "Fleet search API only supports searching a single index. Found: [" + Arrays.toString(indices1) + "]."
                         );
                     }
-                    sr.setWaitForCheckpoints(Collections.singletonMap(indices1[0], waitForCheckpoints));
+                    if (waitForCheckpoints.length != 0) {
+                        sr.setWaitForCheckpoints(Collections.singletonMap(indices1[0], waitForCheckpoints));
+                    }
                     final TimeValue waitForCheckpointsTimeout = request.paramAsTime(
                         "wait_for_checkpoints_timeout",
                         TimeValue.timeValueSeconds(30)