Fix test for empty permissions

Hello,

The F-Droid website currently shows an empty permission list when an app requires no permissions (example). However, when reading the file _layouts/package.html, it seems that the intention is to display no permission list at all.

This merge request removes the permission list for apps that require no permissions by changing the line:

{% if package.uses_permission != "" and package.uses_permission != nil %}

to:

{% if package.uses_permission != empty and package.uses_permission != nil %}

as shown in the Liquid template language documentation.

Results:

• package with an incomplete permission list: before merge / after merge (no change, permission list is still incomplete)
• package that requires no permissions: before merge / after merge (empty permission list no longer shown)
• package that requires permissions: before merge / after merge (no change)

Note that this merge request may be closed in favor of !446 (closed) or !1122 (merged) which add a "No permissions" message instead of removing the permission list.