Apply suggestion from @DaanHoogland

Author: DaanHoogland

server/src/main/java/org/apache/cloudstack/dns/DnsProviderManagerImpl.java

@@ -171,7 +171,7 @@ private DnsProvider getProviderByType(DnsProviderType type) {
      */
     private void validateDnsServerUrl(String url) {
         if (StringUtils.isBlank(url)) {
-            return;
+            throw new IllegalArgumentException("URL cannot be blank.");
         }
         UriUtils.validateUrl(url);
     }