Fix redirections to empty tabs (#290)

Whenever href (url) attribute was omitted or had no value
the click action would do nothing. With no URLs defined and
`target="_blank"` attributes, which were recently added,
anchor tags now redirect users to new blank tabs (side effect).

This PR solves the issues by adding conditions
before applying `target="_blank"` or in some cases by
ommiting entire entities (e.g. the view_certificate button).

layouts/partials/cards/skill.html

@@ -1,5 +1,5 @@
 <div class="col-xs-12 col-sm-6 col-lg-4 pt-2">
-  <a class="skill-card-link" href="{{ if .url }}{{ .url }}{{ else }}javascript:void(0){{ end }}" target="_blank">
+  <a class="skill-card-link" href="{{ if .url }}{{ .url }}{{ else }}javascript:void(0){{ end }}" {{ if .url }}target="_blank"{{ end }}>
     <div class="card">
       <div class="card-head d-flex">
         {{ if .logo }}