fix: nullptr excepation

Author: leonardo-doyensec

doyensec/detectors/cve-2019-0192-apache-solr-unsafe-deserialization/src/main/java/com/google/tsunami/plugins/detectors/cves/cve20190192/ApacheSolrCve20190192.java

@@ -254,8 +254,8 @@ private boolean isServiceVulnerable(NetworkService networkService) {
       payload = this.payloadGenerator.generate(config);
       // Use callback for RCE confirmation and raise severity on success
       if (payload == null || !payload.getPayloadAttributes().getUsesCallbackServer()) {
-        logger.atWarning().log(
-            "Tsunami Callback Server not available: detector will use response matching only.");
+        logger.atWarning().log("Tsunami Callback Server not available");
+        return false;
       } else {
         oobCallbackUrl = payload.getPayload();
       }