conf: Avoid formatting empty <capabilities> element

conf: Avoid formatting empty <capabilities> element

If none of the 'capabilities' features are enabled we'd still format the
opening and closing tag for the <capabilities element.

The implementation is suboptimal but will be refactored for a better
approach. This is done prior to the refactor to show that tests are not
impacted.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>