added null check due to failing CI

Author: piergm

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

@@ -207,7 +207,7 @@ public ShardSearchRequest createShardSearchRequest(Shard r) {
             numberOfShards,
             searchType,
             source,
-            source.query(),
+            (source != null) ? source.query() : null,
             requestCache,
             r.aliasFilter,
             r.indexBoost,