refactor(sidebar): adds condition for github/twitter username when displaying the icon as a contact

Ensures a GitHub or Twitter username is configured in `_config.yml` before displaying them in the sidebar. Adds a conditional to capture/assign `url` so that it is `nil` if no username was provided. That way when the check for `url` is done for building the link, it will be skipped if no username is configured.

Author: JasonRaveling

_includes/sidebar.html

@@ -54,9 +54,13 @@
     {% for entry in site.data.contact %}
       {% case entry.type %}
         {% when 'github', 'twitter' %}
-          {%- capture url -%}
-            https://{{ entry.type }}.com/{{ site[entry.type].username }}
-          {%- endcapture -%}
+          {% if site[entry.type].username %}
+            {%- capture url -%}
+              https://{{ entry.type }}.com/{{ site[entry.type].username }}
+            {%- endcapture -%}
+          {% else %}
+            {% assign url = nil %}
+          {% endif %}
         {% when 'email' %}
           {% assign email = site.social.email | split: '@' %}
           {%- capture url -%}