docs: fixes an error during doc compilation

Building the docs raises an error:
UndefinedError("'logo' is undefined").

This fix checks if logo is defined.

Author: hutchison

docs/source/_templates/pplnx_template/layout.html

@@ -44,9 +44,11 @@
 <!-- MOBILE TOP BAR OVERLOAD -->
 {% block mobile_nav %}
     <i data-toggle="wy-nav-top" class="fa fa-bars"></i>
+    {% if logo %}
     <a href="{{ pathto(master_doc) }}">
-        <img src="{{ pathto('_static/' + logo, 1) }}" class="logo" alt="Logo"/>
-        </a>
+      <img src="{{ pathto('_static/' + logo, 1) }}" class="logo" alt="Logo"/>
+    </a>
+    {% endif %}
     <a href="{{ pathto(master_doc) }}">
         {{ project }}
     </a>