Empty link in Support us nav item

[gosko]

The Support us top level nav item has an empty link following the intro text:

<p>Make a huge difference to our operations as a public-interest non-profit, and help us to achieve our vision.</p>
<a href=""></a>

When browsing through the menu using keyboard navigation this shows up as a small highlighted empty character.

[NicolaSaunders]

To fix, we need to amend how the dropdown panel is templated, to check whether or not intro links are empty. Currently it is assumed that there will always be at least one intro link.