Add Missing Nullable Assertions in SnapshotsService (#49465)

Just realized we were missing some annotations here which was somewhat
confusing since other methods/parameters have the `Nullable` annotation
wherever a `null` can be passed.

server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java

@@ -1061,20 +1061,21 @@ public void onFailure(final Exception e) {
      * Removes record of running snapshot from cluster state
      * @param snapshot       snapshot
      * @param snapshotInfo   snapshot info if snapshot was successful
-     * @param e              exception if snapshot failed
+     * @param e              exception if snapshot failed, {@code null} otherwise
      */
-    private void removeSnapshotFromClusterState(final Snapshot snapshot, final SnapshotInfo snapshotInfo, final Exception e) {
+    private void removeSnapshotFromClusterState(final Snapshot snapshot, final SnapshotInfo snapshotInfo, @Nullable Exception e) {
         removeSnapshotFromClusterState(snapshot, snapshotInfo, e, null);
     }
 
     /**
      * Removes record of running snapshot from cluster state and notifies the listener when this action is complete
      * @param snapshot   snapshot
-     * @param failure          exception if snapshot failed
+     * @param failure    exception if snapshot failed, {@code null} otherwise
      * @param listener   listener to notify when snapshot information is removed from the cluster state
      */
-    private void removeSnapshotFromClusterState(final Snapshot snapshot, @Nullable SnapshotInfo snapshotInfo, final Exception failure,
+    private void removeSnapshotFromClusterState(final Snapshot snapshot, @Nullable SnapshotInfo snapshotInfo, @Nullable Exception failure,
                                                 @Nullable CleanupAfterErrorListener listener) {
+        assert snapshotInfo != null || failure != null : "Either snapshotInfo or failure must be supplied";
         clusterService.submitStateUpdateTask("remove snapshot metadata", new ClusterStateUpdateTask() {
 
             @Override