Fix [object Object] in staff logs details modal

Author: nlalonde

app/assets/javascripts/admin/templates/logs/_site_customization_change_details.js.handlebars

@@ -1,9 +1,17 @@
 <section class="field">
-  <b>{{i18n admin.customize.css}}</b>: ({{i18n character_count count=stylesheet.length}})<br/>
+  <b>{{i18n admin.customize.css}}</b>:
+  {{#if stylesheet}}
+    ({{i18n character_count count=stylesheet.length}})
+  {{/if}}
+  <br/>
   {{textarea value=stylesheet class="plain"}}
 </section>
 <section class="field">
-  <b>{{i18n admin.customize.header}}</b>: ({{i18n character_count count=header.length}})<br/>
+  <b>{{i18n admin.customize.header}}</b>:
+  {{#if header}}
+    ({{i18n character_count count=header.length}})
+  {{/if}}
+  <br/>
   {{textarea value=header class="plain"}}
 </section>
 <section class="field">