Add null check, remove preceding comma before nobrl

Author: abh1sar

plugins/hypervisors/kvm/src/main/java/com/cloud/hypervisor/kvm/resource/wrapper/LibvirtRestoreBackupCommandWrapper.java

@@ -152,9 +152,13 @@ private String mountBackupDirectory(String backupRepoAddress, String backupRepoT
             mountDirectory = Files.createTempDirectory(mountDirectory).toString();
             String mount = String.format(MOUNT_COMMAND, backupRepoType, backupRepoAddress, mountDirectory);
             if ("cifs".equals(backupRepoType)) {
-                mountOptions += ",nobrl";
+                if (Objects.isNull(mountOptions) || mountOptions.trim().isEmpty()) {
+                    mountOptions = "nobrl";
+                } else {
+                    mountOptions += ",nobrl";
+                }
             }
-            if (!mountOptions.trim().isEmpty()) {
+            if (Objects.nonNull(mountOptions) && !mountOptions.trim().isEmpty()) {
                 mount += " -o " + mountOptions;
             }
             Script.runSimpleBashScript(mount);