Fixes #2008 — Empty red badge renders in sidebar when badge callback returns an empty string

Problem

When a sidebar item's badge is configured as a dotted-path string (callback), the red badge wrapper is always rendered — even when the callback returns an empty string. This leaves an empty coloured rectangle in the sidebar.

Root cause

In unfold/sites.py, when item["badge"] is a string, the callback is imported and stored as item["badge_callback"] = lazy(callback)(request), but item["badge"] is left as the original dotted-path string.

In unfold/templates/unfold/helpers/app_list_badge.html, the wrapper guard is:

{% if item.badge %}
    <span class="...">
        {% if item.badge_callback %}{{ item.badge_callback }}{% else %}{{ item.badge }}{% endif %}
    </span>
{% endif %}

item.badge here is the path string (e.g. "myapp.badges.unread_count"), which is always truthy — so the <span> always renders. When the callback resolves to "", the inner content is empty but the wrapper stays, producing the empty box.

Fix

Resolve the lazy callback to a string first and only render the wrapper when the resolved text is non-empty. Behaviour for literal-string badges (no callback) is unchanged.

Screenshots

Before: empty red box next to the menu item when count = 0.

After: no badge at all when count = 0; badge with text when count > 0.