forms: add empty span to checkbox widget, for custom CSS styles (#25978)

wcs/qommon/form.py

@@ -231,7 +231,7 @@ def checkbox_render_content(self):
         attrs.update(self.attrs)
     return htmltag("input", xml_end=True, type="checkbox", name=self.name,
                    value="yes", checked=self.value and "checked" or None,
-                   **attrs)
+                   **attrs) + htmltext('<span></span>')  # for custom style
 CheckboxWidget.render_content = checkbox_render_content