client/views: fix incorrectly 'checked' checkboxes

When similar posts were found, the anonymous upload checkbox for that
image would become checked, because we treat anything !== undefined
as 'checked', and in post_upload_views.js we set 'anonymous' to
a querySelector, which returns null on failure and not undefined.
Treat null as 'unchecked' to fix this issue, and prevent future mistakes
slipping past.

client/js/util/views.js

@@ -81,7 +81,7 @@ function makeCheckbox(options) {
             name: options.name,
             value: options.value,
             type: "checkbox",
-            checked: options.checked !== undefined ? options.checked : false,
+            checked: options.checked !== undefined && options.checked !== null ? options.checked : false,
             disabled: options.readonly,
             required: options.required,
         }),