Resolve OpenAPI defaulting to a single server

The OpenAPI specification dictates that that an empty or missing server
object should default to a server object of one item with a URL of "/"
[1].

This is reflected in 0.8 of openapi3_parser so the code here is adapted
to reflect that.

[1]: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#openapi-object

lib/govuk_tech_docs/api_reference/templates/api_reference_full.html.erb

@@ -1,7 +1,8 @@
 <h1 id="<%=  info.title.parameterize %>"><%= info.title %> v<%= info.version %></h1>
 <%= info.description_html %>
 
-<% unless servers.empty? %>
+<%# OpenAPI files default to having a single server of URL "/" %>
+<% if servers.length > 1 || servers[0].url != "/" %>
   <h2 id="servers">Servers</h2>
   <div id="server-list">
     <% servers.each do |server| %>