Fix up lock modal wording (#532)

* Fix up lock modal wording

When locking a domain, the "domain" variable is null so we shouldn't
display it.

core/src/main/resources/google/registry/ui/soy/registrar/RegistryLock.soy

@@ -118,8 +118,9 @@
   {@param? domain: string|null}
   <div id="lock-confirm-modal" class="{css('lock-confirm-modal')}">
     <div class="modal-content">
-      <p>Are you sure you want to {if not $isLock}un{/if}lock the domain {$domain}? We will send
-        an email to the email address on file to confirm the {if not $isLock}un{/if}lock.</p>
+      <p>Are you sure you want to {if $isLock}lock a domain{else}unlock the domain {$domain}{/if}?
+        We will send an email to the email address on file to confirm the {if not $isLock}un{/if}
+        lock.</p>
       <label for="domain-to-lock">Domain: </label>
       <input id="domain-lock-input-value"
           {if isNonnull($domain)}