test: don't return 0, at least one request must be added to msearch request

also the maxConcurrentSearchRequests() setter only support positive values.

core/src/test/java/org/elasticsearch/search/msearch/MultiSearchIT.java

@@ -63,7 +63,7 @@ public class MultiSearchIT extends ESIntegTestCase {
         }
         refresh();
 
-        int numSearchRequests = randomIntBetween(0, 64);
+        int numSearchRequests = randomIntBetween(1, 64);
         MultiSearchRequest request = new MultiSearchRequest();
         if (randomBoolean()) {
             request.maxConcurrentSearchRequests(randomIntBetween(1, numSearchRequests));