Fix fleet search API with no checkpints (#79400)

Currently the fleet search API does not work properly when no
checkpoints are provided. It throws an exception indicating that the
number of checkpoints do not match the number of shards. This PR fixes
the issue by allowing the search to progress if no checkpoints are
provided or if an empty array is provided.

Author: Tim-Brooks

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)